On a general note, the YS-D2 has suffered from some pretty serious problems. We had 4 fail on a trip last year...I would guess that Sea&Sea are busy sorting them out, and I know the failed strobes were replaced by the suppliers without any problems.
Here are some ideas for sellers to ensure their security:
Sellers are generally in a stronger position than buyers!
Get a shipping quote that includes compensation and door-to-door tracking and ask for payment via PayPal before you ship the goods. Stress that any customs duties are payable by them (buyers often do not realize this).
Avoid payments via Western Union, Moneygram or the like. PayPal or bank transfer/wire are pretty secure.
Do not ship until the money is in your bank account (not your PayPal account). There have been scams (elsewhere) where sellers have received email that seem to come from PayPal showing payments, but these are forgeries.
Buyers will need to provide a shipping address and other details in order to complete the transaction. Use Google maps etc.ti ensure that the address look legitimate.
Remember you can back out at any point up to the exchange of money.
Please feel free to add any more advice to the thread.