I would never sell a gift given to me. They come from the heart and are symbolic for the relationship I share with that person. My aunt and uncle by marriage - they give me an eggstractor - it sits there and it reminds me of how thoughtless they are. My girlfriend gives me a Todd Rudngren/Jethro Tull concert poster - my parents buy tickets for me to come with the family on a trip to Florida after a lot of stressful months for all of us - these gifts are thoughtful and remind me of how much I care for the individual. Gifts, as just said, come from the heart, and are not things that, in my mind, should be along the lines of hi-fi gear or other such things. I can buy the things I want in regards to my pastimes - gifts are not just physical objects, but also representative of caring, love, and devotion.
Whether a gift is good or bad, thoughtful or thoughtless, it doesn't deserve to be sold or traded. You can fund your purchases with your own hard-earned money. I find it morally wrong to sell gifts given to you by family. If they are thoughtful, they would only give you a gift if they thought you'd like it. One must be respectful of this act of kindness.
I've experienced that many of my friends agree with me on this issue - especially ones from Asian (particularly Japanese) decent. The Japanese, as I've learned, particularly value gifts in their culture and find them to be of extreme importance (I've only had official business with one Japanese man - but needless to say, we traded gifts!). They are seen as being of true value and kindness - instead of how they're seen often in America as merely being expensive items of mostly unthoughtful consideration.
IMO it depends on who gave it and what it is.
Some gifts are especially heart-felt, chosen with great care . . . others are, well, just gifts.
Obviously the former are to be kept and cherished, they are special.
The latter are a different case and can be exchanged without guilt.
Heck some people will even 're-gift' those to someone else