An anemone lives for decades in the wild and generally less than 2 years in our tanks. Most die within a year.
My suggestion is 1) unless you know that an anemone is fragged from another already in captivity, or split from one in captivity, don't buy it; 2) unless you know exactly what #1 needs in terms of light, and care, and your tank can provide for it, don't buy it; 3) unless the person selling it knows the answers to #2, don't buy it.
LTA's can get BIG, well over a foot wide given proper conditions. They can live for 80+ years in the ocean. These creatures do not belong in captive tanks.