This is because the front 3 rows of each market can usually make significantly more than the rest of the trade shops. Also, if you haven't noticed, the size of them is bigger. They are 20 blocks in length as opposed to the 15 in the rest of the stations. It is only fair to auction it as the shop is worth so much.
Giving some insight into the shop ( since I used to own it ) The location is good for "passing traffic" but that is fairly minimal, due to the "newer" tp points. The key as with any shop is advertising, good stock and prices. But as I just found out, KEEP AN EYE ON THE EXPIRE DATE !!!!! I lost a LOT due to a mix up on the dates
