It depends what you are selling. It has to be an item valuable enough to make the shipping hassle and seller’s fees worthwhile. Search for the thing you want to sell, and then hit the Completed Listings filter. That will show you what it has been selling for.

I use eBay to but/sell used bicycle parts and miscellaneous outdoor gear. Also sometimes used clothing if it’s niche or higher-end. Getting screwed by other users is very rare in my experience.

Craigslist is still sometimes useful, though it isn’t active like it used to be. It’s best for things you just want gone: post them in the free section and leave them out on the curb.