Nothing against Walter but, as admin / owner of the site, I am very protective of members. While offerings are usually legitimate, we have seen scammers use these forums for ill purposes.
Thus, usual caveats apply when any new user posts offerings with limited information (photos, details, etc). The Amphicar community is pretty small so it is unusual, though not impossible, for cars to appear where nobody has knowledge of the vehicle or their current / prior owners.
Proceed with caution. If it appears too good to be true it probably is. If you are asked to send a deposit right away, red flag. If you cant make arrangements to see the vehicle, red flag. If you are asked to wire money, red flag. If you can't get detailed pictures, VIN numbers, etc. then red flag. (Hugh Gordon used to ask people to send pics which included a current dated newspaper in the photo).
Remember, there are members of this forum located all over the USA, usually you will find someone willing to go check out a potential purchase on your behalf. I have done so for others. Ted Matthews did an awesome pre-sale inspection on my Porsche 914 way back in 2010.