I have a very healthy, robust one right now, but I had 2 for a very short time a few years back. I bought them at the same time under the assumption that they were both haddonis, and would require the same type of care. Unfortunately, while one rooted into the sandbed within a night, the other refused to attach onto anything. Ultimately, that one died 2 weeks later.
While I had the anemones, I noticed a very slight difference between the two, but at the time, not enough to differentiate them as two separate species. One had a tight, compact tentacle pattern, having somewhat of a "cobblestone" appearance, while the other had a tad more of an "open" pattern. FWIW, the one with the "cobblestoned" pattern is the one that survived and is thriving to this day. Not sure which species is the one that survived (haddoni or gigantea?).
A few things you may want look for before buying....make sure it's attached onto something, no lesions on the foot (prone to infection), no gaping mouth or acontia filaments, "color patterns" (my blue carpet is definitely NOT completely blue) and check to see if it's nematocysts are still strong. Oh yeah, I also have to reiterate what lots of people on this thread have been saying, to be very wary if it was cheap. If the deal is too good to be true, it more than likely is. Anything under $100, I'd definitely pass on, just to be on the safe side. FYI, at the time I got the anemones (1997), it wasn't uncommon for them to fetch prices over $175, sometimes commanding as much as $225. Don't ask how I know this......
HTH and good luck!