The main shops restock every 5 minutes. However, because of rarity or the number of people online, often, they might not restock every time. For example, the costume shop might only restock every half an hour.
As mentioned on the main Making MP page, Learn what times the shops restock. Have a small clock on your desk and work out, with a little practice, the exact second that they restock. Have the shop you want to buy at, open, and then just hit refresh (F5) at the second it restocks.
Only restock items that will make at least 500 to 1000 mp profit. To see what items are selling for, search for that item on shop search to see what the lowest priced item is worth. If you are unfamiliar with what items are worth profit and what items are useless to buy, then check shop search first.
When restocking, you DO NOT have to enter a price. Just type in the code and press enter. However, if restocking the cooking ingredients used for fates, then you will need to enter a price. Do not restock too many of each item as the prices will gradually drop and you may be stuck with a heap of an item that is making a loss. I have made this mistake a number of times myself.
Not all shops are listed below, I will endeavour to find out the times for the others.
Times Ending in 1 and 6 (Example: 1.01; 1.06; 1.11; 1.16; etc)
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Chocolate
Weapons
School Supplies
Glowing Eggs
Minipets
Costumes
Minipet Food
Pearls
Female Shoes
Appliances
Seeds
Times Ending in 2 and 7 (Example: 1.02; 1.07; 1.12; 1.17; etc)
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Candy
Soft Drinks
Halloween Treats
Armour
Bakery
Pizza
Charity Shop
Times Ending in 3 and 8 (Example: 1.03; 1.08; 1.13; 1.18; etc)
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Books
Furniture
Car Parts
Potions
Balloons
Voodoo Dolls
Hair Dye
Stars
Canned Food
Times Ending in 4 and 9 (Example: 1.04; 1.09; 1.14; 1.19; etc)
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Charms
Stamps
Toys
Medicine
CD's
Trading Cards
Magic
Cooking Ingredients
Female Clothing
Lipstick
Times Ending in 5 and 0 (Example: 1.00; 1.05; 1.10; 1.15; 1.20; etc)
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Musical Instruments
DVD's
Tools
Flooring
Computers
Male Clothing
Contact Lenses