sadly there's sites dedicated to buying accounts of all sorts (reddit, x, steam, etc...) that use an escrow-type system so both parties have little risk

It's basically super easy and trivial to buy verified accounts for many many platforms

How can I sell my old accounts?

The trouble is getting caught, and you don't list contact info on your HN profile so any potential buyers would have to leave their email address here and tell you to email them and the people running. Steve are an idiot so any email address that appears there is gonna get blacklisted different connected to a Steam account so you might want to list your email address well throwaway that's not attached to your Steam account account. You might want to list that in your profile. Just saying. I already have a Steam account.