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, live well over a foot wide given proper conditions.
Anemones can live for 30+ years in the ocean. These creatures do not belong in captive tanks.
I will add to the above that feeding an anemone too much or too often can also hasten its demise. They expend a lot of energy digesting the food that falls into them the in the wild or is offered by their hosting clown fish.