From what I can gather, they aren’t selling the ability to link custom domains as your user handle. They are acting as a domain registrar to allow users to buy domains and link them to their handle in one step, then likely skimming some profits off the top. Imagine they sell a “custom username with domain” for $20 per year, they pay the wholesale fee (around $9 for a .com) and pocket the rest. That seems perfectly reasonable to me.