To be honest it sounds to me as though the seller is trying to run you out of time to escalate the case by trying to mislead you with a load of nonsense. You do not need to take the item back to the seller's nominated delivery depot - so long as you didn't select "I Changed My Mind" as the reason as to why you want to return the item then the seller ought to have provided you with an eBay Returns Label for you to use in order to return the item; however, if the seller has provided you with some other kind of label then contact eBay Customer Service, explain the situation to them and ask them to issue you with an eBay Returns Label so that you can return the item. This is especially important if the label that the seller has provided is insufficient to cover the total weight of the item, although eBay ought to be able to sort this out for you if this is indeed the case.
With regards to the fact that you said that you do not drive and are unable to get to the Post Office, try going onto the Royal Mail's website and booking a collection from your home (see the screenshot at the bottom of my reply). If you go online and run a search for this option click on the part where it says "Royal Mail Parcel Collect" and it will take you to the site. You can then book a time for somebody from the Post Office to collect the parcel from you, so all you really need to do is package the item up, stick the eBay Returns Label onto it and once the Royal Mail have collected the item from you and it has entered the postal system eBay will be able to put a trace on the parcel using the tracking number to see what is going on with regards to the return. So long as the tracking proves attempted or successful delivery to the address that the seller provided on the eBay Returns Label you ought to be able to secure a total refund.
In the event that the seller proves to be obstructive and doesn't refund your money voluntarily, or starts making silly excuses in order to run you out of time, escalate the case to eBay. The option to do so will appear wtihin the case details four working days from the date that you opened the case. I would suggest doing this as soon as you get the chance if the seller is messing you around. If you do have to ask eBay to step in and force a refund then so long as the tracking details show attempted or successful delivery of the item back to the seller eBay will force the refund and your money will be refunded to you in full.
