either way yoru good to go- if you wait the 2 year, then you can get the 100 off, if you do the one year, you lose your 100 credit, but do get the discounted price.
I've done both, you have options and i've not had any issues selling a "new" phone on ebay...i think one stip was that i had to have the data for 30 days or so- i can't remember, it was on a treo 650, if that dates this
i've done it on regular phones as well. VZW is also really good about taking back phones your not happy with, never had an issue there either