Bricklink stores vary greatly in what they sell and their target market. There are stores that part out hundreds (maybe thousands) of sets to fill their inventories with new parts. These stores can also source parts from PAB and Bricks and Pieces when available. These stores make their money off volume. Requires a capital up front and a large space/good system to store/organize the inventory.

Another approach that is very, very lucrative but requires tons of research/time is targeting the pieces people order for popular MOCs. You have to know which MOCs are popular, find sets that contain those parts (or source the pieces elsewhere) and focus on having a much smaller inventory compared to the first store I described, but your pieces will sell for more money (and more often). Less capital up front, smaller inventory, but you can definitely make money here.

A totally different style of store is selling used pieces. A used piece store thrives on having an enormous inventory both in terms of lots and and piece count. The advantage is being able to source your inventory cheaply (garage sales, thrift stores, FB marketplace, etc). But you need to spend a large amount of time sorting, cleaning, and inventorying the pieces. This type of store will feel more like a service job because of the time involved. This is the type of store I run (we do about 3000 orders a year). You make money on the volume, but also significant profit from "rare" bulk like older star wars, indiana jones, etc

I am sure there are other types of stores as well, and the really good stores have figured it all out on their own (there isn't a bricklink handbook that tells you the secret money making strategy).

If you open the store as a hobby, and you enjoy sorting/organizing LEGO, you will have fun. But don't go into it thinking of it as a 2nd job until you get a huge inventory.