Slow paced story with unlikable MC.
First of all, the MC grows stronger with a system, as usual, but what really makes a difference in his strengh level are Classes that he equips and not the Merchant System itself.
The System only sells and fuses these classes for him, so for the most part it's there as a way for him to have a cheat class.
The whole Merchant System ends being a secondary plot device that has little effect on the story, except for the purchase of Classes and things that he mindless sells for a quick buck, like breads.
The MC personality is terrible. For the most times he's a gullible beta male that can be bullied by anyone, similar to japanese novels. But sometimes he snaps and becomes some ruthless killer, only to become beta JP Mc again a few chapters later. It's inconsistent and weird. Choose the inocent ***** boy hero or the ruthless killer type, he can't be both. At max he can have character progression and go from ***** to ruthless, but not have a murderous switch like this novel.
The pacing is slow and things happen very slowly, with a lot of meaningless written content in between. For a novel with a system that depends on class levels and killing monsters, there is very little action in these 60 chapters I've read, but there's a lot of random exposition or explaining of things that didn't have to be explained.
I don't know if this is the first novel the author has written, and if it is I commend you for the idea and writting style. Unfortunately the good ideas you had for this novel were not properly explored.
Try to organize the MC personality and character progression with adjectives and see if his actions are not contradicting his personality for example.
In regards to levels of power, if you use a game like system where experience is needed, you must use this system often or it becomes meaningless. He has changed classes several times but only had one battle were he got the experience and it was quite dull, he only used an OP skill and killed everything. You could even make him win exp when making merchant transactions and this would made your novel quite better.
Anyways, I hope you can organize your ideas in a better way in the next one.