The MC has a cheat ability, one that properly utilizes the plant magic. Without it, he might only hasten growth of bulk-grow low-quality produce.
The spells make the difference between a house and a bundle of sticks.
But even so, these spells will do little to help against a battle mage. So he's not really able to trample over everyone and anyone.
As for him not haggling, he has no need to haggle when the initial price is good. When the other party is this impressed, they will give a good price because this is a golden opportunity, because if they give a bad one, they may lose their deal, which, when they're dealing in such exotic wares, is unwise.
But as people wise up, he will have to step up his game.
Also, cats are cute.