The canned answer for questions about mileage is:
Depends on how the bike was ridden, and if maintenance was done regularly/properly. The best thing to do if you are interested, is go look at the motorcycle. Give the thing a good inspection and see if the seller is willing to let you ride it over to a mechanic.
Ask lots of questions about the bikes history. Look for signs that may indicate the bike has been abused, such as scratches, dents, dings, scrapes.
There is a good sticky in the New Rider/First Bike section that has a checklist of what to look for, and what kind of questions to ask the seller.
If the bike has been well maintained, and not ridden by someone that's abused the crap out of it, 12k isn't that much. Crunch the numbers and it only had about 900 miles a year put on it. Not that much, really. Though that can be a bad thing if it sat around in storage quite a bit and wasn't "layed up" properly.