keep watering it
he has no buyer protection for a pick up item, so for an unscrupulous seller (not saying you are), all the balls are in your court, you have the money and the plant.
I would give eBay a call, explain to them what's going on, cois I do think that if you
a) send 3 emails requesting collection by a certain date and,
b) send a final letter of demand
and if they still fail to collect it, that as you can show that you have tried to honour the contract by making the item available for collection, that you would be able to sell it to someone else (after issuing a refund of course)
I know it's an often trotted out excuse, but sometimes tragedy does strike a person when they least expect it and seems to take on a greater importance than a plant that they have paid for.
eg, a child may get sick, and they don't generally give a lot of notice for you to prepare, (I had a child who went from perfectly healthy at 2pm and at 7:30 that night was told "she'll be dead within an hour, have you considered organ donation". That kinda made stuff like pot plants seem a bit insignificant.
or, a while ago, life was cruising along just fine. I got a phone call that during a routine medical checkup for my father, that advanced cancer had been found. From that moment on, through the hospitalization, surgeries and treatment etc for the next few weeks until the end, I can honestly say that the last thing on my mind would have been a plant.
so, sometimes stuff does happen,?
At least they have paid.
Some people can go their whole lives and never really live for a single minute.
