Just because a title says "Sterling Silver" or "9/18ct Gold" does NOT mean that it is, in fact, just that. You MUST read the description very carefully as it may in fact mean Plated. Even after reading the description, you are still not sure, email the seller, ask for any stamping or hallmark, aslo ask for a picture of the hallmarks. Also, even if the description gives size and weight, re-ask that question, so when you get your reply, you have all the information in one place, this will make it much easier for you to make that decision.
If it is silver, it may not have the full hallmarking on it, it may very well have only 925, this does not mean it is not silver, it is still sterling silver. The same goes for 10kt, this mean 10 carat gold. This is where you have to be careful, making sure that the stamps the seller say the item has, are geniune markings. If you are not sure about the markings, they may well be genuine, but not something you have heard of before, but before you go any further, do some research, after all, it is your money you could be losing.
If after everything, you are still not sure, then DON'T BUY! Ones of the best sayings is "Buyer beware".
At the end of the day alot will come down to common sense, if the price seems too good to be true, then it more than likely is!
This doesn't mean that you won't get that bargain, they are still out there, but don't just assume that the one you are looking at, at this moment time, is the one. It probably isn't.
I get alot of people emailling me, asking questions about stamping, size, weight etc. I don't mine, at the end of the day I know what I am selling, nobody else does, and I, like these people, email for further questions. Any genuine seller on ebay will not mind questions being raised, that is part and parcel of being a good ebayer.
REMEMBER - Always read feedbacks of sellers, this is an excellent reference point for anyone who is thinking of buying, it will either tell you that this person is reputable, or someone you should be wary about.