@sp: why dont you restrict miner to ip address? Each buyer should tell you payment id, email and external ips which will be used. I vote for no fee for us who donated. Easy and safe solution in my opinion.
and if i have 5 rigs and dynamic ip? what can i do?
LOL - same here. Also dynamic IP's!
SP - How about a key that we have to input to start the miner. This way you know exactly who is selling the miner. After a while if it is only newbie accounts then newbies get the fee miner or no miner at all?