check if an imei is blacklisted on-line prior to purchasing

There are no publicly available IMEI databases that I am aware of.

Even if there were, it would be easy for some one to sell you the phone

and report it stolen after your paid for it - the phone would show clean

when you checked, but you would not be able to use it for long.

If in doubt, do not make the purchase.

Buy from Apple or the local Apple authorized retailer to avoid this problem.

Otherwise, you pay your money and you take your chances.

PS: The seller did not necessarily steal the iPhone, but may have bought

it and ran into the same blacklist problem you did, and then decided to

pass the problem onto some one else by reselling the iPhone.

The only reliable place to buy an iPhone is from Apple, one of the supported carriers, or an Apple authorized reseller. Anything else is, more often than not, a dice roll, and you will likely not roll a lucky seven.