Your problem will be that you have used the mower, so it's no longer in the condition that it was delivered in, and the seller won't be able to re-sell it as new/unused. If you had noticed the extra weight before you had used it (isn't 20/20 hindsight great?) then you'd have no problems.
Your best bet (if you can't work things out amicably with the seller) is to raise a SNAD (Significantly Not As Described) claim through PayPal and hope that they find in your favour - in which case you'd get a full refund and the seller would have to pay for the return. I'd expect the seller to contest the claim, though, which is why I'd definitely try to reach an agreement with the seller before raising a claim.