flea market is a online function of live tarkov for players to buy and sell item from each other, spt aki is a offline modable experince of tarkov. So while you could probably find a mod that makes bots imitate players that sell thing and buy things on the flea market(probably will never exist as a mod), otherwise it will never sell because no one or thing will ever be able to buy it. you will have to sell to traders or custom mod traders