I still have trouble with how に and で differ when dealing with locations, and I just stumbled across にて. Are these all interchangeable? How are they different?
In your example, "market" could mean a few things:
So the meanings of your examples can be:
A better example to remember the differnces might be as follows:
The first indicates with "に" the target of your desired action.
Whereas the other will make them wonder if you're completely sane, and if not, why you're even bothering to ask for permission to sit on the floor of the bus!!?
My understanding is:
Your example is a little difficult because one could be referring to a market you have to physically be in （で）, and the other being like an online marketplace ("に"）. But in general, "に" and "で" are not interchangeable: