I will give you my opinion as the highly paid consultant answer:
It depends.
In my opinion, if you can go to a gun show or dealer directly, inspect the weapon in person and find it better than average it's probably worth $100 or more depending on what it is. If you're mail ordering it and you have only the guy's word on it, you're still buying a pig in a poke unless you've dealt with the guy before and know he means what he says and that you are both saying the same things.
I look at rifles on an auction site right now and one guy grades his NRA fine and another grades his NRA very fine. I look at pictures of them and see no difference and I assume that they probably ultimately got them from the same batch at the same time. Maybe they are not being dishonest, afterall both dealers, and my, evaluation are subjective.
If you have to return a weapon you are probably out shipping both ways, maybe a transfer fee and maybe a restocking fee, minimum. That doesn't include your time.
If on the other hand you inspect a rifle in person and find it's in better shape than it's comtemporaries, the dealer knows it's worth more, it's worth more to you, and you can probably resell it for more if you chose to in the future.
If you're just after a good shooter, almost every mail order dealer I've seen says if you fire a milsurp weapon, it's yours, no returns. So the only thing I'd ask if this person is going to provide a target is, "Can I then return it if I don't get the same results?". Otherwise, what does the target buy you? I've seen some offered with targets and I'd say that they are an indication of what that particular model of weapon could do, but I personally wouldn't place any more stock in it than that. If you've really found a dealer that will sight-in a milsurp weapon for you that may be different. The ones I've seen provide targets from the manufacturer or some where else, who knows what's happened to the rifle since then?
Re-cosmoline? Are there any dealers that do that? Why? Unless they are filling a military order to some third world country. Personnally, if I was paying a guy extra money and he un-comolined a weapon and he offered to re-cosmoline it, I'd insist he didn't.