Firstly, you need to ring PayPal and say to them “You revoke any consents (actual and/or implied) which eBay may attempt to rely to access the funds, in connection with transaction, and if they ignore this instruction you will lodge a complaint with the Financial Ombudsman Service.  Then follow up by giving intruction in writing (email will do),  Also privide  them with copy of the response you sent to eBay.

Then, through the dispute console provided by eBay send them something like this –

The item was listed as pick-up only.

On (insert date), which is the date arranged for pickup, the buyer inspected and accepted the goods.  As such, in accordance with that part of the Sales of Goods Act which pertains to the purchase of goods on description, having inspected and accepted the goods. they cannot now reject then.  Instead their only recourse is request a return of the Goods on the basis of a change of mind, and whether the seller decides to accept or reject that request is at the absolute discretion of the seller.

In any event, in so far as it is relevant, the eBay Money Back Guarantee clearly states - “Not Covered - …when the buyer arranges pick-up.  The Guarantee also states change of mind returns are also excluded from the Guarantee.

I light of the above I, on (insert date), placed PayPal on notice, that any consents on which you (eBay) may rely to gain access my PayPal account in connection with this transaction are revoked, and should they ignore those instructions ,  a complaint will be lodged with the Financial Ombudsman Service.