eBay are attempting to make me break the law

I buy two electric bikes. I want to exercise my UK right to a return and refund within 14 days. The seller delays for over a month and refuses to arrange a safe pickup (UK law prohibits transport of lithium batteries in the same package as the bikes). eBay puts it on five day hold but then removes the hold and escalates the case to the high value team without notifying me. The seller becomes aggressive. I have a call with eBay and they can't tell me why the hold was removed and escalated, why I wasn't notified, or when the new deadline is. I ask what I should do. They say to return the bikes "In black sacks using one return label". I point out that this is illegal under UK law. They say they can't discuss the law with me and end the call.

Both the seller and eBay are attempting to make me break the law. What can I do?