I wish it was easy to sell a used computer but it's really not. First of all you want to make sure you clear everything off of the hard drive and reset the computer back to its original state. Maybe with a new operating system on it.
But then when you go to sell it? You could try to sell on eBay or Craigslist but I really suggest that you don't use those venues, because they're really full of crooks. Trying to scam you into stealing your money.
I have found a lot of good websites for selling used computers but not all computers have much resale value. If you have a used computer that you picked up at a local office supply store or just on a whim you bought a computer. It's unlikely that that commodity tablet, laptop or desktop computer has much value.
On the other hand there are some computers that maintain their value well. Including most Mac computers and some Windows computers like Microsoft Surface and Surface Pro. These specialty computers maintain their values better.
In terms of selling used electronics, Gazelle.com is one of my favorite
sites. And you can use it to sell laptops and phones and tablets. I just this morning came across a website named sellyourmac.com which seems to give the best prices on used Mac computers.
In any case. Do look for valid, well respected websites for selling your used technology and avoided general purpose sites like eBay and Craigslist.