There are many web sites that can help you learn more about eBay. While I use and like eBay, another alternative is Craigslist which is free and is eBay's biggest competitor.
Buying on eBay
eBay distinguishes between used items and refurbished items. However, the only difference is that only large sellers at eBay, e.g. Dell, BestBuy, etc. are allowed to sell refurbished items, whereas you and I can only list these items as used. You can read more at https://www.pcworld.com/article/828603/ebay-resellers-cry-foul-on-refurbished-rules.html
PayPal also offers purchase protection if you don't receive your item, or if it is significantly different from what it was supposed to be. PayPal will reimburse you up to $30 when you return a product with which you are not satisfied. PayPal does not give you this benefit automatically. You must sign-up for this service prior to using it, by visiting this site. PayPal has a long list of caveats that come with this service.
You can learn more about top rated sellers.
Selling on eBay
There is lots of helpful information about selling on eBay.
You will be using the Quick Listing tool to list an item you wish to sell. However, eBay is in the process of converting everyone to the New Listing tool. Eventually everyone will be converted to that.
You may wish to learn more about Reserve Prices.
Here are directions to list your item in a second category
The fees you are charged to sell on eBay can be a bit complicated. You can learn more about ebay fees at this site
If you have a problem on PayPal you can use their Resolution Center to resolve it.
When you sell on eBay, they hold the payment from your buyer for a period of time. The length of this delay depends on many factors.
If you wish to print shipping labels for an item you sold on eBay, you can easily do this at home, which offers several advantages.
If your item doesn't sell, you can learn more about relisting your item.
You can learn what to do if your buyer does not pay you for an item.
If you do a lot of selling on eBay you may wish to consider using the Sellers Hub
If you don't want to sell on eBay yourself, you can have a company do it for you.