If it looks to be in reasonable shape and it works, I think that is a good price. The lifetime of pool cleaners is typically around 3-5 years. That is a good enough discount off the new price that it seems worth the risk that it is actually used far more than they describe. Also, if it was actually used heavily it will show a fair bit of wear, which would be reasonably obvious. The only significant risk is that it doesn't work at all. If you can cover that by either testing it before purchase or getting an agreement to get your money back if it doesn't work at all, then you should be fine.