First of all, PlayerAuction functions like this:
1. What was the feedback of the guy who sold you this account?
2. Did you buy insurance?
3. Do you register your email/have the IP address of the guy, last passwords, etc?
If the answer to #1 is 'high' then talk to them; if they don't do anything talk to PA. If the answer is 'low' then move onto step #2, #3.
If the answer to #2 is 'yes', then open a dispute with PA. Their insurance policy is pretty ass tbh, and even if you do buy its doubtful whether you'll get the 'insurance credits' from them. They basically just try to harass the seller to fix it (and if he ignores them then nothing). Ergo I avoid the so called 'insurance' unless its offered free by the buyer.
If the answer to #3 is 'Yes'...which it should be (Sorry, but this is completely your responsibility) then you should both open a dispute with PA and try to recover. When buying an account always get the previous information from the buyer (he might lie), but you can always take note of the 'last ip logged', changed email, set several different passwords, set recoveries, etc. He still might be able to recover if he has all his information.
If all your options are exhausted, I would recommend trying to get all his information from PA (and if it fails to recover), maybe try retribution and get the account banned. You can do this by offering proof to JaGex of person selling (providing full information from PA). This will stop him from re-selling to the next victim/maybe teach him it's not that easy.
Some extremely important tips about PlayerAuctions; It is not really 'safer' then anywhere else (DO NOT MAKE THE MISTAKE OF BUYING FROM SITES LIKE GOLD4RS/FUN WHATEVER THE FK THEY ARE NOW -- THEY ARE JUST RESELLERS = 2x as unsafe AND 2x the price). All RuneScape trading should be done on 'reputation.' Look at someone's feedback; if they have 10+ (preferably 100%) feedback trading medium-high value accounts over 2 years +, then I consider them legit. That being said, avoid 1-9 feedback people on PA(especially if their feedback is not from account selling/low) who are new to the site and suddenly 'quitting [Insert game], selling my precious [Insert highly recoverable and valueable account] because I need money for [insert bullshit excuse].'
I also feel like I need to clarify one more thing for others. People who are mass selling accounts: 'Account 100% hand-trained' are 100% liars.' These are the type of people who generally might have high feedback (especially the 1K+ FB sellers), but their account support is extremely poor (say you get banned the day after you log into account, or account is inexplicably recovered). This is relevant because these are the type of people who are mass-botting hundreds of accounts on often-times similar IP's (the people you see training straight off of tutorial island on goblins with bronze short sword/wooden shield at level 40). They run off the principle that even if a lot get banned, a few will squeak through and be worth it. The consequences of this is, if your account is created on an IP that is constantly being banned, it might eventually be flagged causing increased watching of your account/a permanent ban within a few hours of first time macroing.
Best of luck. I apologize for your loss and wish you the best.
*Just for clarification; I am not an account buyer; I just have sold ~20 accounts on PA (and a lot more on/to other sites) and this is how it works.*