Used car manager here.
Used cars are a different animal but you can usually tell how much room is in a car using a few key metrics.
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Age. This is the biggest factor. If it is over 60 days old I can almost guarantee there is no gross left in the car (unless they have a lazy / careless used car manager but that’s rare). Check deal rating on Cargurus. Generally you want to sign somewhere in the “good deal” or “great deal” categories. I use cargurus religiously when pricing my own cars as it leads to faster inventory turn. Lower price = front page, front page = huge increase in srp and vdp, huge increase in srp / vdp = lightning fast inventory turn.
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Check price drops in Cargurus. If they have dropped the car more than 2/3k they are likely at cost. They will drop their pants fast on a used car if they have room because it’s all about inventory turn. Nothing else matters. I average about $1,500 front end gross per car. Not much, but I usually do about 80 cars a month and F&I brings total PVR (profit per vehicle retailed) to $3,100 on average. So I bring in 3,100 x 80 = $250,000-ish / month in gross profit to the store from used cars alone. Not to mention I pay full retail on recon (which isn’t cheap) so the store profits a large amount there as well. Obviously it fluctuates but most people tend to have delusional expectations when it comes to discount on used cars. Occasionally we have 1 or 2 $10,000 profit deals but those are unicorns that usually come off trade under allowance.
Tl;dr - Look at age / price drops / Cargurus deal rating and comps within 200 miles. Avoid anything with accidents or damage on a CarFax at all costs. Get a PPI done by a 3rd party, people beat the shit out of GTRs in my experience.
Edit: Also try and finance through a credit union, a large chunk of that $3,100 average that finance gets me is marked up interest 
Happy hunting 