I sell on both eBay and Mercari. Of the two I prefer Mercari because the fees are cheaper and they have a "go on vacation" mode, which eBay doesn't (unless you pay for extra features). If you sell on eBay do not do auctions, they almost always underperform.
Facebook Marketplace is also an option. I haven't sold off of it, but I've bought off of it.
My main advice is to pick up a postal scale, truly invaluable in figuring out shipping.
Also, if you live in the US, you can arrange to have your mail carrier pick up your packages for free (as long as you've pre-paid the postage. You can purchase and print off shipping labels online.) Saves a lot of time!