Selling copyrights ?

Hi !,

I'm being asked (now for several times) to hand out the copyrights of the designs I make so people can resell them ... I'm sure I'm not the only one who's been in this situation, and up to this point I've said no, since I don't think it's fair of me to work for $5 ($4) and people resell the work for $20 or more ...

But I was wondering, has anyone sold the copyrights ? if so, what is a price you've considered fair for them ?

Thanks in advance !

d