Since you can only insure for the declared value, if the declared value is zero then your insurance is zero too. Be prepared for a few sleepless nights if you go down that route.
99.9% of the time this is done simply to avoid import duties. This is, of course, illegal, and will be entirely on you as the shipper should TSHTF. Anything in writing from the buyer is going to be worthless in this situation, especially if you signed the Customs declaration.
If the buyer is using a credit card via PayPal they have two layers of protection. You, as the seller, just have PayPal - and I wouldn't trust them to find in your favor should the shipment be damaged, disappear or be seized by Customs. At the very least they are going to want to see what you insured the goods for. If that's zero, well that's what they'll refund you. That said, if you're shipping on someone else's dime (e.g. FedEx account), then this muddies the waters even further when it comes to PayPal's varies policies concerning refunds should a problem arise.
If you really want to do this, bank transfer (cash) is the only payment to take, and make it clear in writing to the seller what the full value of the shipment is. Then use their FedEx account and get them to prepare all the paperwork so that you just have to package up the goods and let FedEx attach the label.
That said, there will be a long 'paper' trail (including this thread) of the potential conspiracy to defraud Customs in the buyer's country, so I would just forget the whole deal.
MF gear is hard to sell these days, so it's really tempting when you get a single buyer for the lot, but I would personally insist on doing everything above board, with full insurance, or look elsewhere.
Jim