What category was the items you were trying to sell? Assuming it was a legitimate eBay email, there are certain items, especially if you are selling them cheaper than they cost, and they are brand new, they will make you prove you own them in an effort to stop people from selling stolen goods on eBay. For instance, if you sell an Apple i-pad, brand new in the box, for a buy-it-now price of $500, and they cost $1000 brand new (just making up numbers), they may ask you to prove you own it. It's usually initiated by the company (such as Apple in the example) because the company wants to protect their brand and keep the cost up.

Not sure of your specific situation, but it can happen for legitimate reasons.

If you are selling silver rounds or silver strikes, they shouldn't ask that.

Dave.
SS-1232