Important questions to ask when purchasing vintage/antique jewelry to distinguish whether it is a dud or a treasure.
Is it what it is claimed to be?
Always ask before you look at the price tag! What is it?; How old is it?; What is it used for?; what is it made of?; whether the stones are genuine; whether the parts are original - and anything else that you need to determine what the price of the piece may be. Once you have all your questions, then look at the price tag.
Often dealers will not know all the answers, but it is more impressive if the simply say I don't know, then if they fabricate an answer. If you happen to ask "Is this gold?" and your answer is "it may be" assume that it is not.
Is the piece in good condition (preferably perfect)?
Look the piece over carefully. Look for solder marks, differences in color or a hinge that looks like a new style in an old piece. With antique engagement rings (or any other finger ring) check that they are not so worn out that they are destined to crack or that they haven't been improperly sized (with soft solder that leaves a greyish dark mark).
Try to move stones with your fingernail to make sure that they are tight in their settings and that they are held in by metal and not glued on (don't buy rings that show glue around the settings, as the gold was probably so worn that only glue could hold the stone in place). Overall check that the piece is reasonably free from scratches, cracks or chips.
Generally speaking a piece that has a major repair or a major flaw is worth much less. On the other end of the scale, be prepared to pay top price for the finest.
Beware of anything that needs fixing as most jewelry repair men do not know much about old jewelry and don't like working on them. They may prefer to rebuild it rather than repairing it which would immediately decrease the value of the piece.
One important tip to remember is that only an antique piece of jewelry that is perfect or very close to perfect has any real resale value.
Much of this can still apply to online auctions. When bidding for a ring, do not be shy in asking the dealer all these questions, most of them are only too happy to oblige.