Make sure that the item you're interested in is the most suitable for your needs. Before doing any contact to the seller, compare the rate price with another (item) ads with exact matched specification (eq: Yamaha model, year of fabrication, etc).
Make the seller that advertised the ad is a trusted seller. Be aware that the Yamaha motorcycle ad, you're interested in, might be a scam or some kind a 'used item for sold' fraud.
Negotiate with the seller in public place. Some place that easy to access by both party.
Check item's condition throughly.
Verify all legal related matters. Make sure that the motorcycle is not a stolen vehicle, by check the registration to local authority. Also check whether a insurance or warranty came with the motorcycle