Agree with pole king - I'd read this as shagged, at the very least you'd need a decent service on it, and if it's been standing around for a while the seals may need replacing.
Prices are a lot lower now you can get a super gx390 with all the bits for 1500
Search ebay for q washers and see the price of brand new genuine honda gx340
You taking a chance, a most likely be troublesome offer 300 tops then you've got cash to buy spares/service/etc..
Darran