There arnt Fixed Excange Rates, this Kind of Deals are made for Publicy Reasons (For Example to Featuring Entropia Forum) or to Contribute something to the EF Communety.
EF is no Regular Currency you can Cange into Peds whenever you want. But you can always change RL $$$ to EFD by Donating for this Forum
The EFD auctions is usually when ppl wanna sell something for EFD.
Asking to BUY something for EFD would create too many threads with no replies, so let's keep it at ppl selling stuff for EFD, not asking to buy for EFD.