My thoughts are this would work well in a very small server.
But we have quite a large community here.
I spend quite a lot of time buying and selling things, ain't no way I am going to spend 3 or 4 times as long browsing a bunch of stores trying to find items I want rather than the exceptionally well made search in the online market.
It also stops people from getting scammed as easy as they can very easily compare all current prices on an item they want side by side, rather than potentially not finding the cheaper items in the in game market.
In game there is also no way to easily tell which items are new and which are not.
All in all, in game systems are just too inefficient.