So I'm curious. I'm considering getting this professionally installed. Do you think the installer would buy the existing head unit off of me or at least give me a reduced price?

Not if it's a plain-old factory radio. Professional stereo shops have no interest in radios that say "Honda" on them. About the only place you could pawn off one of those is at the swap meet next to the car thieves.

what would I do with the replaced unit?

Save it. When you trade in or sell the car, you can put the factory radio back in. Improves the resale value.

I'm looking into buying a Honda Odyssey. I can't seem to find out whether or not it has an amp though.

Rarely, if ever, do factory stereos include an external amplifier. Plan on getting one. And while you're at it, upgrade the factory speakers to some really nice ones...

Tony Fabris (http://www.jps.net/tfabris)
Empeg #144