The local pet shop will probably take them when they are an inch long, which is about the size you want to sell them at.
Ebay, local pick up only.
Local shopping centre or library bulletin board and put a colour picture and description of the fish and a contact number. Say local pick up only.
50 peacock gudgeons don't take up a lot of room during the first 3 months and you should be able to sell them at 3-4 months old. A 100-200 litre plastic storage container would be sufficient to rear them up without too many issues.