I've bought and sold several over the last few years. I've found the auction sites to be a bit fickle in regards to price. Reasons for the variation and basic things to look out for; the correct markings are present, the lenses are not cracked and there isn't any signs of fungii inside. Dents to the tubes, particularly the flanges that screw them together seriously detract from the value too. Don't buy one without a case and preferably the strap as well- (these come up, but quite rarely). Either the leather or vulcanised rubber external finish if fine- (personal preference ?) But look for one with as much of the blackening on the tubes as possible, not one that's been polished!
I was buying and selling to trade up to the best example I could find. They are out there in immaculate condition, but be patient or get in and buy one,
just be prepared to sell it if a better one comes along.
I've been tracking the auctions for a few years. Got lots of info on sale prices and condition to fall back on. I'd think US$600 should be about top dollar (our exchange rate from Oz ain't good at present) but some that sell for $350 are too expensive for what they are. If you have to look at an auction purchase, I'd be happy to check out the pics if you e-mail me a link to one you're considering. I won't bid against you. Happy with mine at present.