Buy second hand Impact

Thanks Hynzie,
There is a little difference. My way take about 3 hours but for him take about 5 hours. But that's not the problem. Owner of the gun now is there to do some bench rest with some other airgunner. But his friend will catch him tomorrow and he is the one that want to make this deal. (A bit complicated)
He know me in a local social group but I didn't saw any particular question about tuning or fixing impact in that group.
I'll be there the day after that and want to be a good hostile and invite them to my home. At first I'll fill up both guns (my air tank has only 230 bar in it and I have no time to refill it) then make cup of tee or coffee and talk with them. Then check both gun by eye and after that will start shooting. I think till we start shooting is about two hours of talking and checking both guns.
So if regulator has any problem it will leak?
I have to say I will pay extra $200 and also my gun has 3 extra magazine.
Also I have chronograph and I will take slow motion scope cam. plus 25-30 yard group.

Hi NM.
I can service it and I do disassemble and reassemble some other guns like cricket and huntsman regal. but the problem is if I break something I can't order it because they ship them from Arabic country and people in that country found out buying new one is better than repairing the old one. But they sell impact for $1600 and cricket for $1200 (Yes If you leave in they're country you have more income and no taxes). Only Hatsan, Kral, BSA, Steyr, Walther,Gamo and FWB are shipped legally, other not. So I have to be carful, and that's the reason I always service my guns.
I'm not worry about FX airgun QC but I'm worry about it's owner. Now I'm one of that local social group admin and I always take slow motion of my shooting and that's the reason he interested to change his gun with mine. Maybe he couldn't get that accuracy from impact but he said he didn't like impact appearance.
Also I like to make this trade to gain some experience on impact. One reason is in that group some people ask about power tuning or repairing they're impact and I have to answer they're questions (That's not funny because I have to answer all questions about all PCP over 1000 boxes and Kral too). And the other reason is I have pain in my ass.

Thanks EA,
That was really helpful suggestion. I will corny it and check if it's under 20fps or over it.
I'll check transfer port too to see any kind of defection. I'm not worried about Orings but if some metal things were bend maybe it will cause the air leak.
I'll test 25 group test but if I can I will go somewhere to take a long-range shooting.
Thanks QQ
We. talked about price and 200 boxes for him seems likely enough.
I will check barrel with flash light, headlight, sunlight.camera and magnifier to find any fault.
The huntsman regal that I mention it was my friend's gun. I used it for three mounts. But the reason that I carried it for this long was this:
First my friend bought an Airarms 510 as his first airgun and select 6.5-20 Hawke scope. It had been leaking for a while after some week that he bought it. So with out asking me he sold it. At that time he called me and said all problems with that gun and he asked me what to buy. How ever I told him buy impact or wolverine he bought huntsman regal XL (Here the price of impact and huntsman are same). When he was talking about s510 he mentioned that pellet hitting down the target and I can't fix it, so I found that he don't know anything about holdover.
I saw him two days later. At that time he was nagging about my gun hitting so low. I saw the gun and found that he elevated front of the optic by one inch! he did warp the optic.
So he bought 8-32 by 56 SR Pro with out asking me again. 
After some days he invited me and my wife Saha to his family's garden. He wanted to test his gun with me after a while I saw he shot with gun but there wasn't any sign of pellet but he just refilled the magazine.
So I toke the gun and put a pellet inside with my hand. When I shoot the gun some lead backed to my face from it's barrel (after that I always used eye glass protector). I found out he put several pellet inside it, so we went back to little house there to fix it. At that time all of his family became a gun specialist and his father started to do s.th. He insert a welding rod inside the barrel but it wasn't long enough so he add another one and hit them with hammer to push pellet back. After the first one jammed inside it and he couldn't take out the first rod. He said that he will buy a long drill and drill it all the way down. At that time I said that I will fix the gun at home so my friend gave the gun to me. At home I toke out 13 pellet from the barrel. fortunately the rod went into pellets and didn't scratch the barrel. Surprisingly barrel was in a good condition.
After that experience I never bought a second handed scope or gun. But this time is a bit different because the owner of this impact know that I am not an unskilled person so if the gun has any problem I'm not that sheep to buy bad gun so I think there isn't any major problem but I will 100% carful.

Hi
Thanks every one for helping me. With out that I would run into a big problem.
I tested the gun yesterday and everything seems nice until I toke a chronograph. First shot to second shot was about 100FPS (262m/s & 231m/s if I'm not wrong)! All the shot was like that. But non of them couldn't pass 263m/s.
First I thought that was the regulator but each time regulator back on 110 so with out chronograph maybe I could't find it.
No air leak at all. No sign of screw or s.th like that. Inside barrel was like a glass. both manometer worked fine. no scratches or dens on the body. I think hammer had problem.
So I hangout with my little sister for a day then came back to my town.

Hi QQ.
I didn’t buy it and I couldn’t disassemble it to find the fault out. But at the time of test I had 5 different tins of pellet included fx - jsb - h&n.