Stop accepting gifts with strings attached in the future. That will solve most of the problem right there. You are an adult and your parents should not be putting you in the position of dictating your actions and allowing them control over you. Not sure why you accepted the gift knowing you didn't really want it and could have used the money.
Any gift given should be given freely and without restrictions - if they can't do that, then tell them thank you, but you can't accept it.
And etiquette rules from waaaaaay back say that once a gift is given, the giver has no say in what happens to the gift. So you should be able politely thank whomever and then use/trash/donate/sell said gift without any repercussions.
As for what to do with now, if you made a solemn promise on your word of honor not to sell (which is pretty weird, but I've dealt with stuff like that from my mom myself), then it probably would be best to tell them that you don't use it at all, and you'd like to sell it. If they say you can't, then I'd tell them that you don't use it, don't need it and it will just take up space in your house until it goes obsolete, so it will be very frustrating, and give it back (since it's not really yours if you can't do what you want with it anyway).
Does your promise mean you can never get rid of it? So like 50 years from now, you have to be buried with it? Or just for a couple of years until they forget about its existence? ;)
Or if breaking the promise won't cause too many problems for you personally, do so and then just don't ever discuss it with them.
No matter what you decide, make sure that they understand all gifts in the future need to be freely given without strings. It will save both of you a whole lot of headaches.
