What happens if you buy an item from Mr. Hat at full price?

Solution 1:

Simply put, once you meet Mr. Hat in the Void Crater then you will have to fight him no mater what because there is no way to buy the items he is offering at that time.

Even if you try to farm as much gold as you can, you won't be able to save up enough. The prices he asks for are exactly one coin above the max amount of gold you can carry - 99999 gold. Thus you are forced to leave without buying anything, triggering the battle in the process.

Therefore:

• If you visit that location, you cannot skip the battle.
• If you don't visit that location, you won't fight that battle because you will never meet Mr. Hat in the first place (Void Crater is optional and afaik if you skip it Mr. Hat will never relocate to the other two locations he later moves to)