Have you got any legal expenses insurance with your home insurance or similar? Worth checking if you are covered.
See if you can find a lawyer with a free half hour or legal surgery or similar and try and get someone who has contract litigation experience, preferably with property auction experience to see if you can get an idea on your chances of challening it successfully.
Have you got a copy of what you signed? If not it is difficult to advise properly without. Get as much information as you get - everything you signed, additional terms and conditions, copy correspondence, AIP docs, the more you have, the better they can advice. Make a note of all communications, who said what when how etc...
A good lawyer could be key in this - make sure you have a good chance of getting money back before agreeing fees - and although you say you are broke, if they could get your deposit (or even part of it) back then it'd be worth it in the longrun.