Honestly I don't think you're entitled to any sort of refund. Traxxas overgears from the factory and they run hot stock so the truck isn't damaged in that regard. The scorch marks don't effect how it runs and they're not something that's too easily hidden and you admit to thinking they were paint. But if you don't want it and can get yourself out from under it for the cost of shipping it back plus a few bucks for the sellers hassle and what you changed then I'd dump it back on him. Contact him and tell him what you had to do in order to run it and offer $20 off your refund. You get out of something you found out you didn't like for way less than you'd lose if you bought new and the seller gets a chance to resell potentially for a higher price.

Your 540 motor needs to go into a fairly light vehicle, under 5lbs RTR. You can bash 2wd buggies although 2wd stadium trucks (basically buggy with bigger tires) are a little more suited to bashing with their increased ground clearance. I personally prefer 4wd, easier to drift and if you liked having a 4wd buggy then just get another one. You can never predict when a company is going to come out with a new or revised product. I believe Losi and Associated just updated their 1/10 line so it may be another year before they do it again and those revisions are usually only critical for the top level racers. The Losi TEN MT is pretty cool and definitely a capable vehicle but is most definitely too heavy for your 540 motor.