Does sound high, so I'm guessing it really does only have 300 "racing" miles on it.
Check the chain & cogs carefully for wear.
5 Bills isn't bad if it's nice, it fits the lady, and SHE likes it.
OTOH, I got a box-stock '86 ST400 at a garage sale from a lady who wasn't actually selling it, but she said I seemed enthusiastic enough and she didn't ride it much anymore (or EVER, the tires had the little nubs still)
She was happy to take a single Jackson* for it!
*yes, that's 20 bucks.
So, don't be afraid to haggle!!