Specify in a classified advert that I’ll only accept Paypal as a gift or if the buyer pays the fees? I’m offering free postage FWIW. Had lots of replies with offers less than the asking price – which is fine, but then no one replies when I ask for the money either as gift or they pay fees.

Happy to take cash on collection or BACS transfers and have stated such.

It’s fine to ask, the fees can be quite large.
However, the buyer will factor in fees when working how much they can afford to pay so you may get more interest if you pay the fee.