There are two types of vehicle inspections in Maryland. The emissions inspection is done about every 2 years and costs $14, or $10 if you use the new self-service kiosk. Its only a OBD-II check, but they are savvy enough to rule out recent resets. The Maryland SAFETY inspection only is required when the vehicle is sold, and it is very strict. 98% of the used cars fail inspection, if only for the shop to sell you wiper blades and headlight adjustment. For Maryland, I think the seller is being fair on the demands: it would be a pain to go have it inspected, and he seems to feel that the tint would need to be removed. Window tint is legal if it conforms to certain rules of which I have no idea on the details. If you are really serious, I would suggest only a deposit that will cover the inspection cost. Regarding the "realtor" that only signals to me that he might have you sign more comprehensive bill of sale or other sales contract, and he might be a better than average negotiator. I live in the more expensive part of MD, so inspections are right around $100-125: they are usually about 75% of the hourly labor rate. Make sure you do all of the paperwork required for a legit sale, or you could get raped at the DMV on taxes. Make sure you calculate the taxes and fees upfront- they have gone up a lot around 2014.
As for KBB vs NADA vs Edmunds: my take is Kbb is biased towards auto dealers (high retail, low trade in) , Edmunds is neutral but slightly biased towards the consumer, and NADA is about even and unbiased. There is a reason most lending institutions and the MD DMV use NADA.