Resale value is not determined by new price, its determined by the market. Generally, I won't consider something at more than 2/3 of new price, as I value warranty and such. If it is something I specifically want and would consider buying new, then I might go 3/4, but this is rare, most of my purchases are GAS fueled.
Remember shops are in the business of making money rather than increasing the value of their stock. If they got little money for the trade, they'd not be againing anything other than a slighty less desirable bass (one person specifically doesn't want it, whereas the bass they have one person specifically does want).