Finance companies aren't stupid. I used to deal with these scenarios on a daily basis back in my previous life. They will work out straight away that you are a dealer and then you have no chance.
My advice is firstly, do right by your buyer. Its not his/her fault and they shouldn't be made to suffer. Either offer to buy the car back or negotiate with the finco. They don't really want it back, they would rather have cash, so try and negotiate ( you're a car dealer, you should be good at that !) get the best deal you can and take it on the chin.
Lesson learnt, Hpi et all are there for a reason. Always check the car to protect yourself. if you check and the finance isn't registered, but subsequently comes to light, then the data provider will cover you.
You might grumble at paying a few quid per car to check em, but if you lose £10k because of 1 deal, thats a lot of HPI checks !!