I am starting retail arbitrage and used item resell. I have got about 20 stores to resell from...example (walmart clearance). You can do small items like hot wheels, lego,etc.
Sites to resell are: Ebay, Amazon, mercari, facebook marketplace (very frustrating but a source), mpgh ads to navigate traffic (I would persoanlly not use it due to trolls), blogs if you are popular, youtube if you are popular, websites if you are popular, word of mouth, news paper, tiktok (lmao but its possible), and more!
It's time that is hard to get. Lot of people do 100 and turn it into 300-500 dollar. It is hard though, you will need to consistantly go sourcing DAILY. If you want quick flips than it is possible. However, CONSISTANT flips require daily sourcing from many sources and it might take time so you will need to have inventory built up and listed. Good Luck!
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Quicky is going to be your downfall. Nothing is quick.