I'm on the other side of things at the moment I had things in storage, but when I lost my job they changed locks before I got it emptied ( but before last rent cheque was cashed)
I got some stuff but a lot of tools are missing. But as I stand I have been told to give them 14 days to return the stuff or it will be regarded as stolen and I can get police on the job and go through small claims court.
In my case but I wasn't given access to remove the stuff for a year, you gave your mate plenty of time and opportunity.
Keep yourself right and goto a solicitor before anything hasty is done and see what they say, I t shouldn't cost for a consultation on one hand it wasn't yours to sell on the other it could be classed as abandoned on your property,
Tell the mother due to crash damage (i assume you have pics with it being on ebay) It wasn't worth £500, she probably doesn't know that bit of the story.
Or offer to give them £500 after they pay you £510 for storage, recovery.
In hindsight you should have just towed it to the street and phoned the DVLA