1) You can't prove bad bookkeeping practices without an audit and
2) I am not consenting to an audit if that removes me from 4th Amendment search protections (meaning the auditor can be compelled by the government to turn over info without a warrant, when they'd normally be required to get a warrant to search my business).
No bank I do business with is going to just decide "oh you might not be collecting sales tax, so we are suspending your account and keeping your money for six months." There's no ethical justification for that. I am sorry.
Remember: this money they were using for these purposes is actually /not/ their money, nor is it PayPal's: you don't really own money transferred over credit or ACH for six months, which is the window of time during which the money can be nearly unilaterally taken back.
This is a high risk transaction class, and if they had a normal merchant account they would have been required to do all kinds of paperwork verification, accept super-high fees, and pay massive retainers, and in the end likely have a hold enforced on their money anyway until they were considered a good credit risk (and we really are talking "credit risk" here: PayPal is loaning people money until such point as it clears).
Claiming that PayPal should not be looking into these things is about as silly as claiming that you shouldn't be denied a post-pay phone contract just because you don't ever pay your electric bill: this company is exactly as much of a risk (in dollars) to PayPal as PayPal is to this company.