Buying a boat sitting in Puerto Vallarta
First, who is advising you the sale will not be legal? And not legal in who''s eyes: USCG, who will issue the document? U.S. Customs, who will assess the duty? Mexico, whom you will want to issue you a 20-year importation document?
Second, are you planning on taking the boat back into the U.S.? To California? If so, have you talked with your local county tax assessor to discuss ''offshore purchase'' requirements. It may seem counterintuitive to ask for help from the folks who want to collect your money, but having the rules explained up front is usually a wise practice; I''ve never heard anyone tell a story that they lived to regret doing so. If you want the boat, at some point, back in California, be careful to note what their ''constant use outside the U.S. for at least 90 days after purchase'' guideline really means. E.g. your post here suggests you were going to buy the boat, leave it in PV unattended for blank time, and then pick it up in October. I may be wrong but I don''t think that will qualify for tax exemption in CA - but talking with the Assessor will clarify this.
Give us more details and we may be able to better help you. I''d suggest you also post a more detailed similar question on the CW BB (http://old.cruisingworld.com/forums/genlmesg/index.pl) as there are many CA sailors there and I''ll bet you''d be plugging into some helpful info.