If you're worried about scammers there are some things you can do (or, things that I would do)
1. Before completing the sale have them agree to your own personal terms and conditions which state "No refunds". Of course this would likely scare off legitimate buyers who think you're a scammer, but hopefully provision 2 and 3 would put them at ease and assure them that this is a formality to protect YOU from scammers.
2. Offer to have them come collect it from your home or, should they not live too far away, offer to go to their home and help them install it. If they come to yours you can show them the card working in your system then hand it to them knowing it works, if you go to them you can be sure that if it DOESNT work on their system when you get there it wasn't their fault
(excluding obvious technical problems such as them not having the right PSU etc)
3. Should the above not be possible, and just for good measure even if it is, film (if you can) and document the working condition of the card before you remove it, the removal process and posting it. This way you can assure them, and perhaps any authorities should anything go wrong, that when the card left you it was in working condition.
4. Money first, then you post the card/Cash at pick up. Don't send them anything till the money is in your possession.
Hope that helps!