Take it completely apart and sell it piece by piece on E-bay.
There is a small demand for stock parts. Most people upgrade to aftermarket when something breaks.
Look at completed listings on E-bay for specific parts and you'll find out what it's worth.
Don't forget about all the time it takes to tear it down, post it up, ship it out and the pile of parts you'll have left that didn't sell.
Condition also means a lot. Has it been sitting? Rusty? Immaculate? How are the tires and wheels?
Post up some pics (use Photobucket and put the img link in your post) and we can give you a better idea.
Why would you have to pay taxes on it?
If he pays you the $400 won't matter, you'll have $4,000.00.
If you get the bike it will go in your name, not the business, so I think you are safe either way.
Could it be one of these Alba ball joints? The Alba looks angled, is yours strait? If it's not the right one they will refund your money.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=170601894300&viewitem=
There was a post on here a while back about VW and other automotive ball joints being used.
http://bansheehq.com/forums/index.php?showtopic=52038
If you removed the good one from the other side you could match up thread size that might help too.
Where are you located? Maybe you can find someone local with a Banshee that will let you see their bike.
Most everything really only has one way to go without binding. The wiring rides along the top right (sitting on the bike) rail of the frame and all the cables tuck behind the radiator.
Everything goes through the small square hoop (sometimes it's been cut open) in the front of the frame.
Getting a manual is always a good idea.