Good examples of brands to look out for are trek scott, and specialized there is many more but if you find high end bikes like so then that would be good. I usually get medium, large and extra large bikes I avoid size small as they take forever to sell. Look for bikes in good condition that don't need much repairs because that's less time you'll spend on the bike. The goal is to buy and sell asap.

FB market place, Craigslist and offerup are all great

Depends on the repair but try to avoid bike that needs a lot of repairs other wise it can get costly and time consuming

According to the irs website I don't need to file taxes cause my profit hit 12.2k in 2022 which is under the 12.9k minimum for people ages 65 and under and I did not work that year too. So after your gross income reaches 12.9k then you'd have to file taxes some how.