As a shop owner, I just want to warn you that generally, items that you sell will get a lot more exposure at the auction, that at the shop.
If you have hard time selling something through auction, chances are, it will be even harder to sell it at the same price through shop or a shopkeeper at the apartment.
You really should only invest in any kind of retail property (like a shop or a shopkeeper) in order to
(i) Save on auction / listing fees. You pay 0.5 PED flat + 5% of markup to sell on auction, and only 2.5% or so of the mark-up through shops.
(ii) Pay the fee / sales tax when you sell the item, and not when you put it up for sale. When you sell through auction, you pay when you list, but when you sell through shop, you pay tax/fee only when you sell something. With a shop, you can avoid the loss in case the item doesn't sell at your original price.
(iii) Change prices at any time at no cost. You can't change the price of your auction item without re-listing (and paying the fee again). When you put something for sale at a shop, you can change the price as many times as you like at no cost.
But selling at a shop will NOT bring you more customers than selling at an auction!