Logistics question

Hi, I am new so apologies for my naive-ness. So, let’s say I order bulk of a product, let’s say “barstools”. I buy 100 barstools. I go on Amazon and see a listing for my same barstools and if I sold it for that listing price, I could make a nice profit. If I go to sell this (either FBA or FBM), what makes sure my product will sell as opposed to whoever is behind the other listing? What if that person has 500 in stock? I am confused with this concept so any insight is appreciated.