FWIW, guys I'm new here but I do lots of deals on a well known knife forum, I've bought and sold thousands of dollars worth of custom knives and have learned some things along the way.
Always use paypal goods and services. Never use the gift option. Ever.
High quality photos. If i'm sketchy on someone I usually won't do the deal, but I may ask them to take a photo of the item with a current magazine turned to a specific page, a newspaper, or some other easily attainable item. This is not to burden the seller, but simply to provide me with assurance they have the item.
Follow the rules of paypal, don't ask the buyer to pay the fees. Thats not what you signed up for. It's a service and they deserve to be treated with respect. Frankly, a seller who shows this little simple lack of integrity is probably someone I don't want to deal with anyway.
Shipped insured, signature required. Again, it's a service and yes you have to pay for it.
Good communication is key. I don't really care to talk to someone by phone, email leaves a track record.
Ask for references and check with them. Most people have had some sort of track record on the internet at this point.
Be realistic, if it's too good to be true.... well you know the rest.
Again, I'm brand new here, this is just some things I've done to keep my small business afloat and years of buying knives on the second hand market. If anyone wants to check me out. I go by Rustysocket on bladeforums. I make custom pens. My real name is Scott and I'm pretty easy to find.
If I don't go with a new PCP gun ( which I likely will for my first) eventually I will make my way into the used market here.
Take care, and trade safe.