My advice is to read a lot on this site to get an idea of what specific pins should cost. There are lots of great people on here that have decades of experience, and their advice can really help to not get burned on your first pin by someone flipping a project.
The best advice I received was to not buy a project for my first pin. You want to be able to play that pin as soon as you get it home! Struggling to get something working when you are new to the hobby can be very frustrating, especially when you don't have another working pin to go blow some steam off on!
Go in person to play the pin for as many games as it takes for you to get a feel for the condition of the pin. As Doug mentioned above, a good 20 minutes isn't unreasonable.
Be patient. They made lots of these pins, so if the deal isn't right, wait for the next one. Another better one always seems to be posted for sale right after you just bought one!
You will undoubtedly have more questions, so fire away. This a great community, and people like to help.