Im more cases than you think... a power seller who suddenly goes bad is one of two things, something serious has happened (which may include death or pc stolen etc) or worse and hardest to detect: account compromised. One of my best friends recently got scammed in the UK buying a wii. The seller had over 400, zero negative and had lots of feedback for lots of wii's sold in the past.
Long story short, in one 2-week swoop, he managed around £15,000 (30,000 USD) going straight into his bank account from 360's and Wii's bought by lots of people.
What happened was the original seller had his ebay account compromised, paypal removed and instructions to wire money into a bank account. Now, i myself, seeing 400 feedback would not hesitate and go ahead with it... but after a week of not turning up, my friend contacted him, the fraud guy even replied and said it definitely was sent... he waited another week, by then it bacame obvious that it was a scam, since loads of negatives flooded in.
Ok, word of advice that has so far helped my friend one step closer to getting his money back:
A) Contact someone who he has BOUGHT something from, ask nicely if they would give you their address. Do a bit of research online and you may find his home phone number. Give them a chance, maybe something bad has happened. If they say account was hijacked, it is now a police matter.
B) Paypal are responsible, if they are going to handle money, they must be willing to co-operate with authorities and give the IP address and bank information of who is accessing the money.
C) The bank where the Paypal money is being transfered to and/or withdrawn from must have an account holder. He is liable since it is his account and so too is the bank. They must cooperate with Cops to name the account holder, address, and check video of anyone withdrawing cash from ATM.
Nobody who steals money through the internet is untraceable. With some detective work, you can get addresses, names, phone numbers and maybe even a photo...
Let me know how you get on.