Hi Cris, and welcome to the forum
Regarding what products sell best, it's one of the most popular questions we get and also one that it virtually impossible to answer. It's easy to give you a hot selling item if that's all you want, iPhone for example, one of the hottest consistent selling items on the planet. Of course having that information is a good start, but that's are good as it gets.
Now you have to work out how you could compete selling iPhones, especially on a platform like an eBay.
The hottest selling items are the most competitive markets of all and you have to go up against some BIG TIME sellers who are buying in massive volumes and selling at very small margins to create successful sales.
See my point. It's not about what the quickest selling item is, it's about what you can sell quickly with the all factors taken into consideration.
Regarding how much time you need to spend, that's easy to answer ... ALL OF IT!
Research is king in this game, you need to research products, suppliers and your competition. Then you need to keep doing it to stay competitive if you do manage to get an edge, because the moment you stop, someone will take your business right from under your nose. You're entering the most competitive business on the planet!