I would say online selling is probably one of the most popular ways of earning during Covid at the moment. With more and more people working from home and not getting out as much the Health & Fitness niche with the need for home equipment I.e Skipping ropes, Dumbbells, Pull-up bars etc has boomed!
The best way to make money during quarantine would be to do things online in my opinion. Selling things via ebay or amazon could be one solution, but it might be only short term if you don't have much supply. Something more sustainable would be, if you have special skills that you could offer other people doing from home would be the best way. Another way to make money online, which many people started using lately would be to start streaming. If you are playing a new game regularly and are good at it, you start streaming on twitch, mixer or youtube.