I would like to sell some of the domains I have collected over the years. What is the best platform to do that?
I would be interested in an answer as well. I have tried Flippa, BrandBucket and BrandPa - so far not very successful.
Came across https://www.domainholder.io today on BetaList. Looks really cool, but I guess it doesn't handle actually trying to sell your domain.
There's basically two types of customers you can sell your domains too: 1) end-users, and 2) resellers.
Selling to an end-user usually yields the highest price. However, it can be tricky to find them depending on your domain name. Try googling for the words in your domain name and see if any businesses turn up that might be interested in the domain. Reach out to them. The ideal situation would be for a potential buyer to reach out to you. For this it helps if your contact email is available on your site and the page implies it's for sale. A parking service like DomainHolder mentioned by @maheshj567 or Sedo.com can be helpful for that.
If you want to sell your domain soon then you'll probably end up selling it so a reseller (i.e. someone who then goes to find an end-user and adds a healthy margin to the price). https://sedo.com is probably the most popular market place amongst domain resellers. In addition to the sites already mentioned by others, I'd also check out http://namepros.com
Having said all that, I should add that in my experience many makers hold on to worthless domain names. Year after year you renew domain and now you're invested. Sunk cost fallacy comes into play and you don't want to get let it go. Has anyone ever reached out to you asking to buy it? If not, and if based on the homepage it's clear that it's for sale, then there's a decent chance it's a worthless domain. You can still try selling it of course, but if you don't have luck at any of the mentioned market places then it's probably best to cut your losses and let the domain expire.
Another good way to get an estimate of a domain's worth is https://namebio.com – there you can look for recent sales of similar domains. Filter by the same TLD, similar length (or exact length if it's a very short name), and search for the keyword in the name, or synonyms. If any sales of similar names pop up, those prices will give you some idea of what people pay for a domain like that.