My thing is vintage and pro musical instruments. I’ve become very good at knowing the value of such things. I like to buy musical instruments off of ebay that are poorly described/photographed but have good profit potential. Of course, just like one of your answerers, I use very good photography and excellent descriptions. I’ve also learned how to do minor repairs so I can buy an unplayable clarinet or sax, and make it playable often with very little time invested.
I also hang out and antique and collectables stores, this is an education for me so that i get a really good feel for how much people will pay for stuff I find at yard sales. It’s been fun. Nowadays, I rarely sell anything on ebay. I have other people (TRUSTWORTH people) that do that for me and we split the profits. Not bad, eh?
Best of luck.