Although you'd have to ask futbin and futhead to confirm, I believe general consensus is that they use a ton of different accounts. Easily affordable for huge sites like them that probably make a decent amount in ad revenue.
Without a ton of accounts, I don't believe there is a way to check the whole market. Even just limiting it to golds would be difficult. However, you don't need 1 request per auction. Just per search. You might be able to get decent info if your search were something like Gold/LaLiga/ST. I don't believe the API would return everything that matched that, but it would return enough results to help build some market data. And that would be done in just 1 request.