Players over 30 years old are very difficult to be sold but it is still possible. There are always many injuries near the end of the season. So some managers who have urgent needs for replacement may take them. But you should be hard to try to sell them repeatedly. I suggest you starting to sell them assp and not to wait for start of season because you do not know when the manager who need your players will appear. Maybe you lower the asking price to minimum as a incentive for the buyers.

I don't believe there is good method to sell old players. So you should avoid to keep them.