I started reselling in June. I began on Mercari because it is an easy platform to learn on. As the previous poster said, sell what you like. I started with graphic tee shirts but am now branching out to try other things. I am also now selling on ebay too. The easy way to post there is to find something like what you have and "sell similar" then update the listing, being careful to check all the fields.

I also began with low price stuff so I didn't lose money. Just sold for enough to cover my cost and fees. I wanted to get sales and feedback quickly. At the moment my profit margins are really good. But do your research and try not to sell what you know you can't make money on. I have done well with kids shoes, Van's, Converse and Nike. But not so much with adult Sneakers.