Not at all, far from it in fact. Check with the seller about the general condition as previously mentioned, also if there has been any work carried out, how many owners, original packaging, etc, etc. All useful to know.
Determine the payment options - PayPal is good, but if you can, make the payment (if the value is over £100) using your credit card through PayPal. Then, if there IS a problem, PayPal may be able to help, but if not, your credit card company may assist.
I must admit, I can't understand the cash on collection rationale when better and more secure options exist. You've no means of getting your money back short of going to the small claims court for one thing after relying on the seller's goodwill. Paying by PayPal and/or credit card is far more secure in the event of a problem. Note a Debit Card provider won't help you out in many cases; a minority do, even if they carry the Visa or MasterCard symbol, forget it. Use the Credit Card for the additional peace of mind.
The website may not take responsibility, but the seller must describe the item accurately if you purchase in good faith based on that.
Lastly, if you know the amp well, I'd be inclined to go for it as you will more or less have a good idea of what you are buying. Try and talk to the seller first though if you possibly can.
Good luck!