Oddly enough, this one buzzed the net a few days ago too.
http://www.cycleexif.com/yeti-ultimate#more-6873

These bikes are truly a thing of beauty, not that I know that much about MTB's since I'm more of a road bike guy, but the pics of both the bike in this thread and cycleexif blew me away. I'm a bit partial to the campagnolo equipped though, but that's my addiction.

As someone with a little training in both writing and sales I can help you with some advice, maybe, in contribution to the valuable hints already given above.

-structure your pitch: don't present all the info in a solid block - make clear divisions between the history of the bike and vital sales information like condition, year of build, parts etc. There's a lot to be told about this bike so it's worth your while to summarize the vitals in a header. Put everything in neat paragraphs with clear titles in bold.

-Don't be too nervous about your writing talents. The piece above is very readable in itself! Maybe not as a sales pitch in it's present form, but the build history could be added as an attachment for the buffs and collectors, many guys here and elsewhere love to read these stories from behind the stage. Pictures of the workshop, the process etc. from back in the day are welcome too! It all adds to the provenance (and thus the value) of the bike.
The story about your friends should definitely be included - it is a charity auction after all.

-even though this post probably already did it, make sure the collectors of vintage mtb's know about the sale. It's a unique piece and it shouldn't end up with an uncaring buyer who won an auction that was rushed because of the financial situation of the sellers.

Good luck with the sale and best wishes to your friends.