See this if the units are worth anything-- Dell Trade-In Frequently Asked Questions | Dell US If they are too old or don't qualify--Donate the units to Good Will and get a receipt for a tax deductible donation.  
Can someone explain the trade-in program - their seems to be a lot of pros & cons to how it works. I have more than one unit  (maybe three or four)I'd like to trade in or donate to kids who need a computer. I can't pack up every computer using a Fed-ex label. I was told Goodwill excepts them for dell at certain locations - but would this still be part of the trade in program and would you still get credit for the trade in? Just a little confused on the procedure here. Thanks
Dell Trade-In Question