I wouldn't say they were rare. I recall seeing them in stores back in the day. They might be undesirable in the sense that anyone who might want to buy it probably already has an original Super Soaker 50, but some people still might buy it anyway. The performance of the Super Soaker 50 isn't that good compared to even modern water guns (which don't perform that well, I should add). In short, the SS50 is neither rare nor particularly desirable, but you could try to sell it if you'd like.
The Drenchenator, also known as Lt. Col. Drench