When approached about your copyrights, it is wisest to think in terms of licensing them rather than selling them. An artist should never forfeit his copyrights (as opposed to Moral Rights) without being separately and well compensated. When an artist finally reaches the point of relative success, often late in life, he seldom owns much of his own work and must compete in selling it with those who purchased earlier works at much lower prices. His earlier works can actually adversely haunt him years later. He still has to methodically paint each piece and wait for it to be sold albeit at a higher price, in order to be paid. His only real advantage for himself and his heirs at this time may be his ownership of the copyrights on his whole body of works.
Your copyrights are actually more valuable than a patent to an inventor and this is why you should relinquish it to no one.
Within the context of what copyrights are intended to do, no one else really needs to own your copyrights, they can license them and you can still make money off them that way. Everything can be achieved by license without forfeiting ownership.
Remember, even if you're promised great things and high commissions in trade for your copyright, if there's failure to follow through, that copyright is lost forever to you. It is just as easy and far safer to license the copyrights for a certain project or time period and still retain ownership of it for yourself and your heirs. For this reason every artist should have an automatic minimum $100,000 price tag on the actual ownership of each copyright as a starting price for bargaining. This may even be too cheap in the long term.