I say make the buy price higher than the sell price again so there is room in between for chest shops!
Ex. Oak logs are sold at /warp shop for 1000 per stack. You can sell oak logs for 500 per stack at /warp shop (doesn't need to be this extreme of a difference).
Now smkorpi selling oak logs for 750 per stack stands a prayer of making money
Basically charging extra for the convenience.