Well you can always restock at night time, its usually when all the neopets players are asleep.
And you must know the items where you are restocking at or else you'd be losing NP when you sell it. Be sure to have the shop wizard at your side and check it if your unsure what the price of the amount is.
And with DSL, you shouldn't do so bad. I restock sometimes...perhaps if I wasn't with dial up, maybe I can restock better.
A good choice of a place to restock at is most bookstores. Each purchase makes some profit as long as your good at haggling.