Once a wire transfer has posted to your account, it cannot be reversed or undone by the remitter (the person paying). That is why a wire transfer is the only way a seller who wants to avoid problems (fraud, chargebacks, etc. - see below) should accept payment, and why payment by wire is by far the most common way that commercial transactions involving real sums of money are done (and the only way they are done for large commercial transactions).
As for PayPal, okay, okay, I'll come clean. The above posts attempt to explain that buying something (such as an amplifier or LP's) via PayPal in conjunction with a typical credit card permits the buyer to process a credit card chargeback in the event of fraud or where the goods do not conform to their description. In such cases, your credit card company essentially reverses its payment to PayPal for the amount of the transaction, and PayPal is left to collect from the seller (or you, the buyer, if they can't collect from the seller). If you as a buyer process a chargeback, you need to anticipate that PayPal will attempt to draw on your checking account linked to your PayPal account in order to cover its loss, so you must immediately close your PayPal account and/or arrange for your bank to deny any attempted draws on your checking account from PayPal (this may require account closure, depending upon the bank). It may be that the "fine print" of your agreement to use PayPal states that buyers will not use chargebacks (or if they do, promise to indemnify PayPal), but it would not be cost effective for PayPal to enforce these rights except where it involves more than $15,000-$20,000 (i.e., it's too expensive for them to hire a lawyer to sue you if you process a chargeback). Of course, PayPal's fee structure takes into account that 0.0001% of their customers are sophisticated enough to use chargebacks, so cry no tears for them.
In any event, this is the reason that (i) I refuse, as a seller, to accept PayPal under any circumstances, as an unscrupulous buyer can do a chargeback simply because he does not like the product -- a wire transfer is the only way to protect yourself and get deal finality, and (ii) as a buyer, I always use PayPal in conjunction with a credit card, as I can do a chargeback to negate fraud or handle unscrupulous sellers that lie about the condition of goods they sell. In fact, I buy everything I can with a credit card, as the ability to do a chargeback allows me to reverse problematic purchases (for the record, I have never, ever had to do a chargeback, but I have that option if I put it on a card). One cannot, generally speaking, do chargebacks with a debit card, incidentally.
