You can try to track similar items to what you want to buy for a few weeks, to get an idea of the going price. For example I've been watching Harmony ResoTone (Bakelite) banjos for about four weeks and almost all of them sell for between $100 and $115. So if I get serious about buying one, I'll bid $115 on a likely one and probably get it, and probably for somewhat less money.

Rick, some sellers specify "US bids only", others don't. So with them you should be alright.