Worked in the motorcycle industry for 8 years (sales). The larger dealerships will take it on trade but it will not be worth the tax credit. Sell it on your own on Craigslist or in the Cycle Trader and use the money towards your new one, used ones are always in demand! Also, going into the dealership with cash down vs. trade-in will get you much farther! There is a massive Polaris dealer in Enumclaw that could be worth the drive when you're ready?
A couple tips when buying your new one:
1) If it's one that's high in demand don't expect much of a deal.
2) Freight, assembly and prep fees are 95% bull! Just padding for the dealership...out of the crate it takes the shop about 30 minutes to 1 hour to have it ready to roll, so maybe an extra $95 for that?
3) MSRP on atv's only represents about an 8-14% mark-up over cost for the dealer so don't expect to save thousands. If it's a previous year model or demo, the factories allow for that by increasing the dealers 'hold-back' which gives them more room to wiggle!
A good deal on a new atv would be no freight and prep fees and $500-$800 off MSRP depending on model?....if you can get that you shouldn't lose much sleep over the deal

Good luck!