Hello Liberation and welcome to forum!
I also purchased a GS500F at the beginning of this season as well.
Here are a few things you want to take a look at and ask about the bike.
1) Call/Text/Email ahead and ask about the basic of the bike.
i.e. a) Is UVIP or Safety Certificate sold with the bike or provided by the owner. b) Is there any mods/upgrades done to the bike?
2) Ask if the bike has been dropped before? It is usually a 50/50 chance it has been.
It is difficult and rather expensive to install a frame slider for this bike (approx. $100+)
3) Ask when was the last time the bike received the following:
Engine Oil + Oil Filter Change
This will give you an idea how well the bike was maintain by the current owner.
Front and Rear Brake Pads
Check out the wear on the pads. If they need to be replaced use it as leverage to negotiate the price lower.
Front Brake Pads are about $50+ / Rear Brake Pads are about $40+
This was my rear brake pads after I brought it home.
Chain + Front/Rear Sprockets
Examine the chain for rust and slack. If it needs to be replaced you know the deal.
Check out the Sprocket teeth (or rather the rear sprocket - since front sprocket will be covered). Are they worn out? Need to be replaced?
Below is how you can pull on the chain from the rear sprocket to see how stretched out the chain is. If you can pull it? Replacement is needed (approx. $50+)
Brake Fluid
Examine the brake fluid for front and back. Does it need to be topped off? Or has it ever been changed? If it is the same brake fluid since 2006... Replace it ($8+)
Tires
This is important. Check out the condition of the tires. Are they worn out? Are they dried up and cracking? If so that is easily $100+ per tire.
If you can ask the owner to not start the bike until you get there, that would allow you see first hand how the bike starts up.
Take note on the RPM after it has been warmed up. It should stay around 1200k RPM-ish. When the owner has it on choke the RPM should climb up to 4000-5000k RPM for 30seconds to 2mins.
Carburetor
Ask when was the last time it has been cleaned, because that is work for you to do or someone else.
Valve Clearance Check
Ask when was the last time it checked? This stuff can affect the performance and gas mileage of the bike.
That is all I can think of now, but take a look at this site:
http://wiki.gstwins.com/
Get to know the GS500 and understand how to some of the maintenance is done.
P.S. Why doesn't the bike run right now anyways? Is it something serious with the bike?