everymist

This is a bit weird but I actually enjoy cross-dressing pets. Though it's never something I'd bring up on the main site I feel as if it allows a pet to express more personality than just being constrained to a single type of clothes. Because that's frustrating and I, personally, feel that gender identity shouldn't really constrain my pet. He will be fab no matter what. :)

The Magic Shop is indeed very competitive- and no wonder, with it carrying morphing potions that can be sold for millions! The Stamp shop is the other really big one. Weapon and defense shops can get pretty busy, since lots of the rarer weapons sell for A LOT, but if you learn the stock well, you might be able to sneak in and get some that will give you less of a profit.
As said, the Igloo Garage Sale is a really good place to start. Most of the items in there will let you turn a profit (there are still some that you'll find yourself cursing for buying, but not as many as in normal stores). Books and the Pharmacy are pretty good places to start as well. The Pharmacy might be easier, just because it has less stock, so it'll be easier for you to learn which ones are profitable, and which ones aren't.
 
Most of all, be patient. You won't be the best restocker over night. You will mess up, and buy something at a loss. You will mishaggle and lose out on an item. These are normal things that every restocker does- it's okay. Just keep going. If you find yourself getting frustrated, take a break. Just come back to it another time.
:) Good luck!

Igloo garage sale can be a good place to start. Or even the secondhand shoppe (thrift shop), just to get practice quickly scanning items and picking good ones. For IGS, I started off buying an item I presumed was profitable (often books are a decent bet), and then going back and SW'ing the other items to see what I should have grabbed (and what I could be glad I didn't grab).