You moved the car excise-free as you already owned it in Slovenia, so its a "moving good"
@OP,you have to own the car for 2 years outside Poland to avoid excise duty
This is one of the complicated parts so I don't want to get in to it, but basically it was Urzad Skarbowy who said "ok, let's forget about paying excise duty".
Unbelievable, but of course I said no problem.
I wouldn't have thought selling the car with Slovenian plates would be a problem as I think the extra hassle and cost to a Polish buyer is minimal (plus Slovenian roads are excellent compared to Polish).
If I did sell it and used my Slovenian address on the sales contract, I am just worried that I would still have to pay the 'customs debt', as I'm not really sure if my car is in the system, so to speak.
I mean, someone has a file on my car in Urzad Skarbowy, and someone stamped the excise duty document at Urzad Celny, but as I didn't complete the registration, maybe it won't show up on anyones computer screen when a buyer goes to re-register it.
If the customs debt is minor than I'm willing to take the hit.