You won't be able to sell everything to the Pokemart which is likely what you're experiencing. Items like the Comet Shard will not be sellable to the Pokemart. Due to this some items are worthless. Items that have value for players i.e. Berries can be sold on the Ultra Market (/market) but for a whole, items that are only useful for selling to Pokemarts in the base game won't really have any value on our server.