Shops
Here's some tips for shops, both buying from others, and selling in your own. Shops are essential, but I wouldn't make a shop too big, cause it's a waste of neopoints to upgrade. Plus if you are at the point to where you have to increase your shop size past 30 (with 150 items), you might have too much inventory, meaning your prices are too high.
If you have a gallery, that's great. Make a new account for it. You don't want your gallery items taking up room in your shop. Galleries are great by the way, because friends donate free items to it, but the down side is that you probably can't sell those items without feeling guilty, or having your friends wonder why you sold their gifts.
One last thing about "shops", is your inventory. PUT LOTS OF JUNK IN THERE. I usually have dozens of junk items, so the chance that the pant devil or grundo master or whatever will steal something valuable is reduced. Usually once I get a really expensive item I'll put it in my shop, in trades, or to re-sell in auctions. There's always a chance you'll lose the item randomly, but having lots of junk items reduces the risk.
How Shops are Cataloged, and How to price your Items Accordingly
Saving Time Pricing the Items in Your shop
Overpricing and putting items in your shop