Don't know how credible Deltapoint is. Seems a bit random to me.
Surely the current system is:
See how much they're advertised at, deduct your margin and prep costs and see how little you can get it for.
It really doesn't have to be too scientific.
If it's a nice car with good spec, history and condition, it'll sell on its own.
If it's an ordinary car just like everybody else has got, it will also sell.... but only at a price.
The skill is not in the selling, it's in the buying.