I lived in UK for some time and used to sell my stuff online via websites like Ebay and Gumtree...
I would expect there to some equivalent in Thailand...
Where does one sell stuff online in Thailand?
As is usually the case in Thailand, just because you expect something to exist, doesn't mean that it does. The ebay.co.th domain name has been registered here in Thailand for over a decade (originally by Sanook, and used to redirect to one of their pages), and now just redirects you to a page in Thai about selling stuff as export.
There are a few local sites for buying/selling online in Thailand. One of the more popular is Kaidee, although it's in Thai. It is, however, an excellent site. I have sold laptops on there very successfully.
The ever-present Craigslist seems to be a thing used by foreigners more than locals.
Thaivisa have their own area for buying and selling, Thaivisa Classifieds.
Of all of these, the only one I have used myself is Kaidee, but you definitely need to be fairly proficient in written Thai language to get the most from it.
BahtSold is actually great because they target both thai and foreigner community. Plus, they also have expanded territory in other SE Asian countries. Make sure that you include at least 1 photo. There are option to do things like bump your listing or showcase your listing for additional fees.
@Scott Earle already mentioned ThaiVisa classifieds which is mostly foreigner audience and it looks like Kaidee is mostly Thai. No one mentioned another thai electronics hangout, which is Pantip. I do not know the rules for listing there if you actually have to have a physical shop at the Pantip plaza or such.
The question was about online selling, but keep in mind that in Thailand certain items can be offloaded to 2nd hand shops or sold via word of mouth quickly: Motorbikes in good condition, undamaged cell phones, tablets, etc are in high demand.