What's it worth?:

2015 Q70
55k miles
Sport w/ Technology pkg.
Private owner, wants $23k.

I haven't looked at it yet so have no other details. I was thinking $21k but maybe I'm a cheap skate.

Search autotrader and see what they are going for in your area, Q70 and M are not too common so you might have to look in a 200 hundred mile radius. Condition is a huge factor too. Some people say like new and it has dents and scrapes, scuffs in the interior, stains etc.

They are going for around $29k in my area, but they have just under 30k miles and CPO. If the said car was in really nice condition is it worth $23k?

No , cheap skate your not . You could get there , $21k or less by doing what "Ritchie" does :

Take a long ,long ,long-long time looking at the car .

Shake your head a LOT , Squirmy facial expreesion , occasionally explaining in too vague terms for what you've sized-up the "Mark's" knowledge about cars - tell truthfully , but do EXAGGERATE shortcomings.

Negotiate by repeatedly suggesting this & that makes it a SORT of P.O.S.

Remark how ( Like in Caddy Shack ,Rodney to Ted Knight-the hat!) Nobody but a Dork nerd Zombie be caught Dead w/ IT , in your own "Kind words " of course.

Ya can't stand the COLOR - but you'll "put that aside".

Tell'em how starting with tires and ending with the Hula girl in the back window - NEEDS REPLACING
very soon !

This is evil but - throw a chop garlic in the vent that's EASIEST to get into later- Tell'm CAR SMELLS.

"NHTSA has TCB's & RECALLS up the Ying-Yang. " And DO NOT FULLY EXPLAIN NHTSA !!

Every third sentence out your mouth = this thing really ........... is.......... really...a..

This is a good one - on test drive - when they're not looking - 35 mph - Jerk the wheel HARD L-R ,
then ask THEM HAPPENED ???

Look into oil filler opening , facial expression like - you know you just stepped in a PILE , then make
some SH$!TT up .

I won't give away ALL the secrets , those ought to get you at least down to $19K .

I'm guessing it's a V6? The prices depend on the drivetrain as well.
I think that's a fair price, but if you like it, it makes sense to see how negotiable the seller is.