I think you should always be cautious when buying anything used
'Buy the seller' is a very good way to view transactions like this so consider who you are buying from as much as what you are buying
Don't post any details of the deal here on here but feel free to PM as I manage the classifieds on here and have a few suggestions I'd share privately in terms of what to look out for and a few things you should ask
I'm not sure the Amp500 has been in production for 7 years yet so maybe the amp is newer but I could be mistaken
Shipping costs will be a fair bit so the price seems a little low to me but sometimes there's a bargain to be had
As for PP fees make sure you are sent an invoice and don't do a friends and family payment as you wouldn't be covered if something were to go wrong