In my opinion you really need to be able to test the card to know if it works well. You can do this after receipt with an online purchase as long as you have some recourse with the transaction, eg. paypal disputes.
In person, if you don't have a system to run it on then you can't tell by looking at it. If you know the person, or can return to their home, and they acknowledge it's subject to the card working fine, maybe it's ok. If this is a "meet in the park" random stranger it would have to be a real good deal to be worth the risk.
If you have a test system you may want to be prepared to run a test depending on how picky you are. You might test it's ability to overclock as long as the seller allows it. A bootable flash stick with a ready to go mining setup may be a good idea.
(I actually run my rig off an Ubuntu Live CD image installed to a 4GB flash stick. After booting I just went thru the Ubuntu Headless Install tutorial on this forum. It boots and starts mining on it's own and I can ssh into it to check/alter the config. With a monitor attached you can also use that as a terminal. When you boot on a new system you have to do the aticonfig init cmd and reboot for it to know about the new cards installed.)