I've found most sites accept them if they're done well enough (i.e. no obvious joins or differences between the individual photographs).

I've sold a few, including one to a local magazine, but they're a rather specialised area. Most people seem to want them as banners across the top/bottom of pages, and there isn't so much of a market for that.

I did some boston skyline ones, almost all the sites took them and are selling well on at least two of the sites

Do what you love, who cares if it sells!