It's definately a Cameroon Sailfin chameleon (Chamaeleo cristatus) based on the description. They are a little known species with horns, a back sail, and a tail sail. Theses are the only chameleon species with a definite sail, which rules out Panthers, Jacksons, mellers, and Nosys. Some Vieled chameleons are sold under the name Sailfin so beware of imitators. I've been raising and breeding chameleons for some years now and have come across this species a few times. It's very interesting and i would suggest buying one or two or three, $55 is a good price(normal prices are between $80-$100). Their care is pretty similar to most chameleon species but they are notoriously hard to breed. Unless they have a serious disease properly cared for, they could become healthy. It's probably wisest to keep each individual isolated because they have a tendency to be aggressive, especially among females.