Chris - each state is different, but some general rules apply. As a private seller (versus a commercial dealer), you should not be held liable for anything the new owner does after the sale (note that's should not). Here are some steps you can take to keep you on the should side of the equation.
1. Make sure payment is a bank check or cash. Even with a bank check, try to do the transaction during business hours so you can call the issuing bank and verify. There are a lot of counterfeit checks out there that may appear to clear, but will bite you up to two weeks later when your bank presents it for payment to the issuing bank. Usually, you can get a feel for a buyer (i.e., if they try to do this remotely and ask you to cash a check for more than the price amount then ask you to pay for shipping out of the surplus they send - run away!!).
2. Sign over the PA boat title, any trailer title and engine title (if applicable) only AFTER you have received payment and verified legitimacy. Be sure to fill out the title and list the price, the buyer's name and his full address (most have room for this information).
3. Draft up a bill of sale that separately lists as different items the boat, motor and trailer. The elements of a bill of sale should include:
-- Your name and address as "Seller"
-- Buyer's name and address as "Buyer"
-- The purchase price for boat and trailer (list each separately or do two BOS's)
-- The identity of boat, engine and trailer by manufacturer, model, model year and vehicle identification number or serial number.
-- A statement that the Buyer has had the opportunity to inspect the boat, engine and trailer and agrees to purchase "as is" without any warranty from Seller.
-- A signature line for both you and the Buyer with date.
Make two copies of the bill of sale(s), have the buyer sign each, you sign each. Give one to the buyer and keep one for your records.
4. Provide the buyer with COPIES of the registration card for the boat and trailer (some states require you to turn those over - generally, copies will work or at least keep a copy for yourself).
5. Make or retain copies of everything you provide (front and back) so that you have a full set and the buyer has a full set.
With all of that, you should be good to go in any state (as will the buyer). As noted above, individual states sometimes have their quirks, but I have found that the above will generally work.
If you need a draft copy of a bill of sale, ping me off line and I can send you one that I used and that worked for me in buying and selling a couple of boats I've had in the past.
Jim