True, but don't forget that you save on the taxes because you only pay tax on the trade difference. So a $2500 trade in at a 7% tax rate saves you an additional $175. So that is like really getting almost $2700 on trade.
Truthfully, if I knew that I could get the equivalent mower new for $4300, you would be hard pressed to get me to pay more than $3000 (maybe $3200 at most) for a used unit. This is not an indictment of the OP, but anytime you purchase used equipment, you always have the uncertainty of knowing how it was treated by the previous owner.
Given the issues you have had with this unit, I'd think if you push a bit more that you could get the dealer up a couple of hundred on the trade in. After all, I'm sure the dealer wants a happy customer versus someone who will tell everyone that they are not happy with their unit after only 4 months. If you phrase it in the right way, I'm sure the dealer will get the point.
If you can get the dealer up a couple of hundred plus figuring in the tax savings, you would be close to what you could get for it on an outright sale.