Rat Rod nailed it, I just went through this while shipping a complete Spaceliner from Indianapolis to San Diego last week. Greyhound was almost half the cost of UPS and FEDEX. The only draw back is that you have to drop it off at a terminal and the buyer must pick it up at a terminal. If you do go this route be sure to get the buyers phone# as Greyhound will ask for it to notify them of arrival. Also the online quote from greyhounds website or over the phone will be a couple of dollars lower than wath they will actually charge, The lady at the shipping desk explained why but it didn't make any sense to me so I just paid the extra two bucks and went on my way.
Check with your LBS for a box (generally free),make the bike as compact as you can.The longest dimension of the box can be no greater than 48".