Welcome to Piano World. You will find a lot of good advice here by industry pro's and piano enthusiast as well. Some might always agree, but that's okay. It gives you several different opinions to decipher so you can make an educated decision.
Here would be my response to your questions.
1) There are used Yamaha dealers all over the Bay area. I know of a few in Marin County that I could point you to. Craig's List is not a bad place to look. You would simply just have to take more precaution when buying from an individual regardless of how you met them. You find one on Craig's List, eBay, or your newspaper. The point is most individuals would not have the expertise to fairly report to you the condition of their used piano. Therefore you would definitely want to pay a technician to go and inspect the piano before you purchased it.
2) The web question you asked is a loaded question. The piano industry frowns on its dealers posting pianos with prices new on the web. Our industry is a very territorial industry. The fear is if you got on the web and saw a very aggressive price from another dealer somewhere else in the US you would come in your local dealer and expect him to match the price. Without going into a long thesis on the woes of this issue I would simply encourage you to research the forum on thread topics that have to do with buying online to see more details about why you can't find pricing anywhere on new Yamaha's on the web. If you find a price for a new Yamaha on the web it either will be a retail price and not a discounted price, or it will be a discounted price on a gray market Yamaha.
4) Correct. A new piano is covered by factory warranty, and dealer terms of sale. You should be fine if buying new. I would encourage to look for a late model used as well. There are several on the market, and you could save a lot of money.