best shops to restock in are also the hardest. If you are new to restocking, magic will do nothing but destroy your confidence. Stamps can be good for beginners, it takes some time to learn what stamps are junk, what are valuable. General rule of thumb, if it stocks for 10k or more, its r96+ (dont quote me, but its high rarity and always worth a lot). 5k stamps are often profitable, but because of tarla, itll take some learning to know what is junk and what isnt.
As far as big profit, magic is highest volume of unbuyables, but being the most popular shop you gotta be good, and of course lucky.
When I restocked, i like to have a couple "high volume shops" and a few less common shops that rarely restocked, but when they did they were worth 10m+. Here were my favourites:
Magic
Stamp - r99 stamps are some of the most expensive items in the game, good luck getting them, in 8 months i got one.
Meridell Foods - Draik Eggs, stock maybe a couple a day, quite a few people in here, but your basically trying to get lucky and refresh on a rs.
Royal Potionery - Voidberry/Prickly are fairly common. Big item here is Leaded Elemental Vial.
Haunted Weaponry - Glowing Cauldron, DMB etc.
Battle Magic and Defense Magic are other slow, but potentially great places to restock. Scorchstones, TTear, etc.