It won't hurt, but it won't help that much either. The gun shop owner/clerk will probably look up the value in his gun "blue book," or just know the approximate value from memory. He's thinking about how much he can sell it for as a used gun in the condition it's in, and what his costs vs profit will be. If he thinks he can sell it fast, he'll probably pay a little more. If he thinks (or knows) that it'll sit on the shelf for a long time and he'll be lucky to sell it for more than he'd have to give you in trade, he probably won't give you as much. The best you can do is take it in a see what he'll give you for it. I'd be sure to mention that you have the box, manuals and all included accessories that the gun came with. Generally speaking, trading is far easier for the seller, but you're going to get hosed on the value because the gun shop has to make a profit. If the gun will sell for $375, whether you sell it yourself or they take it in trade and resell, then they are going to give you less than that for sure. If I were you I'd try posting it up in the classifieds here. Give it a week or two and see if you can find a buyer. If not, then you can trade it for whatever you can get from a gun dealer.

To be honest, if I had the cash right now, I'd buy it from you. I am in need of another .22-250, and I wouldn't mind having a Vanguard at all. My wife still hasn't forgiven me for buying my last rifle!

Mike