Why exactly wouldn’t it be allowed? I’m not sure what the system is/how it was made as I haven’t visited Trade Hangout for quite a while now. Even so, if they are using some kind of API (i.e Rolimons), how would this be against the TOS?
To answer your original question, they are probably either querying the Roblox catalog API or Rolimons (if Rolimons even has one).