It is up to your specific shop whether they want to deal with used bikes or not. Remember, they are in this for money, and since mark-up is slim, they need to be pretty confident that they can sell the bike for more than they pay you for it.
If they do take a trade-in, you need to understand that a bike is like a car... it loses a pile of value as soon as it leaves the dealer. Don't expect to get a new bike for a few dollars.
As with cars, you might possibly be able to get more for it selling it on your own, if you want to go through that effort. The down side, is that if the shop takes trades, they will give you a solid number, and if you do decide to sell on your own, you stand the chance of not being able to sell it quickly at a higher price... the used market is somewhat fickle. You could of course sell your bike first.
As far as the Slice, from a quick look, it is basically the Dew with a suspension fork, so give it a ride, if you like it, then see what sort of deal you can make.