Go to your LBS and ask them for a box. Heck a lot of shops now offer packing and shipping for people just like you.
They provide the box, the partial disassembly and the wrapping and packing of the bike. Then since they (usually) already have a UPS or FEDEX accnt, they just ship it directly from the shop.
Other than that, many times a shop will GIVE you bike box, and if you ask nice they'll throw in some extra packaging.
If you can find a CANNONDALE dealer. Of all the bikes I've unpacked over the years, Cannondale boxes are bigger and sturdier than others. However, since their boxes are bigger they are bigger than the limits that Fed Ex allows. My Fed Ex will ship them, but I've heard of other places where they will not. UPS is the same BTW.
I've also found FED EX Ground to be them most affordable and Reliable. Also, FED EX is easier to deal with than UPS in the event something gets damaged. Here's a TIP. BUY THE INSURANCE. Not for the price you sold the bike, but for the FULL REPLACEMENT COST (IOW, new MSRP for a similar bike). If you claim the bike is worth $250 and it gets destroyed, they'll only give you $250, but it may be worth $300. Capice? (Think Italian Slang)
Price varies, so state that. If the buyer is concerned about shipping charges, state that if they provide you with a zip code, then you will give them an estamated shipping cost. Don't forget to add the HANDLING Fee that the shop will charge you to pack the bike. I live in FL and have shipped a bike all the way to WA. Costs about $45.00. So, figure it'll be approx 40 to 50 depending on where it's going.
__________________
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger