Errr... I very much doubt it.
Fiio need to make money selling new products, rather than reducing their profit margins by 'buying-back' used goods (which is essentially what a trade-in would be).
I personally feel that Fiio are one of the fairest manufacturers when it comes to pricing, so their profit margin is much smaller than, say, iRiver's Astell & Kern product line.
If you wish to upgrade to the X5, then I suggest you sell your X3 privately, on eBay or here on Head-fi's Classifieds section.
Edited by Mython - 12/14/13 at 12:41pm