they might prefer to keep the money from sales in their paypal account. Some people set themselves a rule that they only buy with money from what they sell
They might be worried that cash might be counterfeit
They might be worried that you’d haggle when you collect, so would rather payment was made first
It might be a scam and theres nothing to collect and the address is false.
Oddly though (and you could perhaps suggest this) paypal isn’t recommended for items collected in person as eBays uses tracking numbers to settle disputes. If you pay by paypal then collect in person – then raise a dispute to say you’ve not received the item the seller can prove they’ve given it to you and ebay will take the money back off them and give it to you – simply because they’ve no proof (and nothing other than a tracking number counts as proof) they sent it.
EDIT – too slow for most of that