I'm no expert but here's my take on it:
The copyrights are still valid, the developers could take action against you if you download or distribute Amiga games. But it is highly doubtful, the programs themselves don't make them money anymore, except in very rare cases. A lot of Amiga developers (EA, RockStar, Team17 for example) have current generation games for the Xbox and PlayStation formats that they will be a lot more concerned about, they won't be checking if a 20 year old game is being downloaded.
As Deafdaz said, drawing attention to yourself by attempting to sell copies online would probably increase your chances of having a chat with the developers.