Okay, not intending to diss your mechanic friend, but "just needing a new battery" is WAY out of line with conventional wisdom/experience for your scooter. I'm not an expert but offer you the following summary based on my knowledge. A ten year old scooter that sat unused will need:
1) New tires
2) New drive belt (even if it currently runs, I wouldn't trust it for 1/4 of a mile ride). They decline from use and AGE.
2) Replacement of ALL fluids
3) Replacement of all hoses/rubber gaskets/lines and filters
4) At a minimum, full fuel tank clean/scrub (quite possibly a new tank)
5) Complete and detailed lubrication (that can be a pain on units that have sat with no attention for a decade)
In other words, excluding use as a parts vehicle, it needs some serious attention before anyone would even think about riding it unless the rider is really an idiot. You also have the issue of it passing any inspection if required in your state.
KBB (Kelly Blue Book) shows a random Los Angels zip code value of under $1K trade-in and $1.4K for a dealer retail unit in excellent running condition. I would not put much faith in those numbers, they are much higher than its worth to an informed buyer, IMO.
Personally, I wouldn't buy it. The refurb costs (unless one does the work themselves) will exceed the ultimate value of the scooter in comparison to other much newer, more refined and properly cared for units.
I'd post on your local Craig's List with all the details as a Best Offer for quick sale and see if anyone bites. You could also consider donating it to a charity and take the tax deduction based off of KBB valuations. Just be sure and print a screen copy of the KBB numbers in case the IRS has issues with it. JMO
You can get better information from a wide range of experienced folks by posting on a scooter specific site. My favorite is the Battle Scooter forum at Adventure Rider (
Battle scooters - ADVrider)
You can also try posting on the Modern Vespa forum (
http://modernvespa.com/forum/) which has a number of Piaggio discussions.