Before doing any contact to the seller, compare the scooter's rate price with another (scooter) ads with exact matched specification. Many piaggio - vespa is modified by it's owner, affecting the price. So there may be wide range of price available.
While contacting the seller, please check that he/she is a trusted seller. Search he/she email addresses in online forum, so you won't be the victim of a serial scam.
When you finally meet with the seller, met them in a accessible place / public place. Check item's condition throughly according to the advertising.
To make sure that the vespa you will buy is not a stolen one, validate item's paper to local authority.