A used 100W panel is going to be less than 100W. Roughly 1% drop a year. Also, the glass can get scratched and translucent, depending on where and how long it was used, further reducing the output. Occasional very old panels can be made with no glass at all, with exposed and/or delaminated leads between the cells. Used panels on Craigs sell for a fraction of the cost for a reason. Still, I think Craigs is the place to go.
You don't tell about controller. There are some very cheap new on Ebay, their efficiency is unknown, but with used panel you are going to have less than 100W anyway.
I don't like those kits with new panels on Solar Blvd. They bundle relatively cheap panel with unknown and unpopular controllers and throw in brackets that you don't need, and some cable that you might not need 'cause it's too thin or too short or you already have a cable.