Yeah, these GS bikes are routinely UNDER $1000 all day long, unless they run in tip-top shape and are in good condition.
(If you're in the USA)
For a gs750, $1300 is high, Unless it's in great shape and low mileage.
But then again, this depends on Your definition of "great shape".
But $1300 is more of a Summer price, so if it's Winter Time where you live, that price needs to drop quite a bit!
If he won't budge much on that price, walk away.
GS's are literally a dime a dozen, you'll bump into them a lot.
A better one will come a long in no time.
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Just because it's running doesn't mean jack either.
Make sure Idle is smooth, no major leaks.
(idle should be around 1100 RPM's i believe, so make sure Idle isn't set real high or anything, because that can be used to "cover-up" dirty carbs...but that's not a big deal really, carbs usually needa get cleaned routinely)
No overly-weird/violent vibrations in the handle bars or while riding.
Are the exhaust pipes rusted, or extremely "blued"?
(exhaust is around $350 for a new setup)
You really have to crawl and look UNDER these bikes, with a flashlight.
Does it look suspiciously clean? Like someone cleaned the motor up, but then you find very dirty oily parts underneath or in corners?
Start the bike up Multiple times over a 15-30 minute period.
Check the Stator Papers on Cliffs web-site, so you know how to look for Voltage Drop and correct numbers that indicate Charging Problems with the bike.
This website will have LITERALLY EVERYTHING you need for your GS.
It's been a complete life-saver for me and everyone here!
http://members.dslextreme.com/users/bikecliff/
Check the Oil, and lights, that kinda stuff.
Usually the lights dim when you Rev these older bikes up, no cause for alarm right away. 90% of the time, you just trail a long the wiring harness, and clean all the connections a long the way, problem solved. But be aware, these bikes DO have a big problem with Stators and Rectifiers going Bad.....so you might wanna bring a volt meter.
But usually a dead battery is a quick sign of the problem....unless the owner hurried and charged it before you arrived.