As you work your way down to 3 vehicles at 3 dealers which ideally are within a reasonable distance from your residence or business.Now what do you do?
If you are considering a vehicle with balance of factory warranty, there should be little concern regarding the "operational condition" of the vehicle. You can inspect the condition of the body and interior yourself, the dealer should provide you with an accident report which shows if the vehicle has been accidented, and history on the vehicle. This information is readily available to any dealer.
These are relatively simple, and straightforward transactions. Keep in mind that you want transparency and clarity at all times. If it gets opaque for any reason you want to at least momentarily step back.
If the vehicle does not have a balance of factory warranty, you need to increase your due diligence. The immediate reaction to is to have the vehicle inspected by a knowledgeable technician, this is time consuming, and most people are challenged when it comes to time. Then you will need to negotiate the "issues" from the inspection, this exchange can become stressful.
The dealer selling you this vehicle must give you a level of comfort from the outset, if you do not have a level of comfort, step back.
If you are considering a vehicle with no remaining warranty, either the selling dealer has inspected / reconditioned the vehicle, and gives you a level of comfort, or you will need to do it yourself, or you step back from this particular vehicle / dealer combination.