The propane tank is a 5 lb tank and likely has the quick release valve connection. Check the date on the tank (it is stamped on the handle) and if it is over 12 years old, use it as a bargaining chip, as a new one costs $50+. The date on the tank likely is a quick way to point out how old the grill is too (obviously the vent code is the best indicator, but not many of us have the codes memorized). Keep in mind that you can refill it yourself or have a kid at your propane station do it and they last a long time.
Check to see if it ignites - again, if it doesn't, you have another bargaining chip - the burner tube often get spiderwebs in it and is easy to clean and get going, so keep that in mind. Basically, point out the imperfections and try to work the price down. Starting at $60 to $75 isn't an insult, and bargain to where you feel comfortable.