Best ways to make a sales transaction so that both parties are secured

Have both parties get a "PayPal" account. Have the person with the gun send a bill (through paypal) for the cost of the sale and shipping. Have the purchaser pay the bill through paypal. The seller will lose 3 percent of the asking price to paypal fees, so he may need to jack up the price by 3%. Do not send the payment as a "gift", to avoid fees; gifts do not have paypal protection.
If he fails to ship, or the item is not as described, you can enter a paypal dispute and get a refund.
 
"addertooth"Have both parties get a "PayPal" account. Have the person with the gun send a bill (through paypal) for the cost of the sale and shipping. Have the purchaser pay the bill through paypal. The seller will lose 3 percent of the asking price to paypal fees, so he may need to jack up the price by 3%. Do not send the payment as a "gift", to avoid fees; gifts do not have paypal protection.
If he fails to ship, or the item is not as described, you can enter a paypal dispute and get a refund.
OK - so I sell a gun, collect PP and ship the gun. The gun gets there, as advertised, but the buyer tells Paypal that it didn't. How, as the seller, I'm I protected? Paypal is "Buyer Protection", that's all.
 
"BigTinBoat"
"addertooth"Have both parties get a "PayPal" account. Have the person with the gun send a bill (through paypal) for the cost of the sale and shipping. Have the purchaser pay the bill through paypal. The seller will lose 3 percent of the asking price to paypal fees, so he may need to jack up the price by 3%. Do not send the payment as a "gift", to avoid fees; gifts do not have paypal protection.
If he fails to ship, or the item is not as described, you can enter a paypal dispute and get a refund.
OK - so I sell a gun, collect PP and ship the gun. The gun gets there, as advertised, but the buyer tells Paypal that it didn't. How, as the seller, I'm I protected? Paypal is "Buyer Protection", that's all.
Tracking and signature on delivery. I think you might even be able to have a declaration of items getting shipped. 
 
I agree with BigTinBoat. I used to sell items on Ebay and elsewhere online. I quit using PayPal and their parent company Ebay for that specific reason. Too many people scamming me as a seller. I even took to photographing and videoing the items before shipping and even then PayPal / Ebay sided with the buyers in every case. I became part of a class action law suit against PayPal for that reason (which they just settled recently). For all of the aggravation, wasted time and items I lost money on I think I'll receive $4.62 after the lawyers get their share. I was primarily selling antique clocks and cameras. I use square for payment processing now. There is some risk no matter how you do it. I person just needs to decide what is acceptable. With all of the online scams and what not even with air guns and other expensive items I have wondered why nobody has stepped forward to act as an intermediary. Buyer and seller agree that transaction will go through "x". Buyer sends money to "x", seller sends item to "x". X verify s that item is as advertised, sends money to seller and item on to buyer. I know and understand that there is more to it (liability, costs, insurance, time constraints, the question of functional verification... etc) but I for 1 would gladly pay a nominal flat fee say $25 on up based upon monetary value of the transaction to be confident that I was not being scammed by someone 2k miles away on an item I am paying several hundred dollars or more for. Just my thoughts and limited experience.
 
It is a fact that PayPal does have some bias which favors the purchaser. However, they are also responsive to photographs/proofs which are submitted by both sides. A seller is likely to prevail if the purchaser is being clearly dishonest. And of course shipping via methods which are tracked, is the only way to fly for a purchase over ten bucks. On expensive items, shipping with insurance is a must. I have had shippers entirely lose packages which had contents which were insured for $350. Dealing with the USPS on an insurance claim is much tougher than handling a dispute through PayPal. My first 3 claims against USPS (for the same lost package) were denied, even though photographs and receipts for the contents were provided to them. They also required a statement from the person it was being shipped to, even though USPSs own records showed the package was not delivered.

​ There are always difficult sellers and purchasers. This is why there is rarely any seller who has a 100 percent positive feedback, they will get that one bad purchaser who deals in bad faith, or has excessive expectations (they expect a used item to be nicer than a new item from the factory). There are also sellers who peddle junk/broken/used items as new. Due to the questionable nature of humanity, any method, no matter how secure, will always have some measure of risk. 
 
The Broker Idea is a good one and I actually thought about trying to set up such a service but ran into these issues
First cost.
It can cost upwards of a $100 U.S to ship an air rifle and other equipment what is being suggested however would mean that there would be two shipping cost once to the broker then again to the customer
Two liabilities
Party A is describing the product in good working order in like new condition
Party Bs idea of like new condition may not be the same as party A
There are ways of minimizing shipping issues
1st signature required helps reduce a lot of issues and I believe there is even a service of ID confirmation though tis does still add to shipping cost. But still you then find yourself at the mercy of the shipper when it comes to damages and they may or may not honor a insured claim.
Another factor is time as PayPal can hold funds for extended amounts of time and any credit card transaction can have a charge back placed on it upwards to 60 days.
And of course trust
At minimum you are looking at about 100 dollars added to cost to broker a deal with an air rifle
And oh I almost forgot some states requires any individual working as a broker for any service to be licensed, insured and bonded
 
I try never to buy from "strangers". I have made 3 purchases and in 2 cases, I did not know the buyer personally but I found out who they knew in the hobby. I also got to "know them" by askng them to tell me their interests in the hobby and some of the other air rifles they own. They told me who they knew and what special interest they had in air gunning. All three times I have sent a check out prior to getting the package and luckily I have received all items without conflict. I was happy to say that Joe Rhea got the check the same day as I got the package. I was not going to open mine until he got his check. If I had even the remote funny "feel" about the deal I would back out. I would never mail to a PO box or large business (unless I could verify they worked there). I would have multiple numbers and their verified personal address. You may want a referal on this forum that you trust to verify they are authentic. Could I still get burned, yes. The best way to eliminate problems is of course to buy new.