I'm amazed that an archery shop would leave a longbow strung that long. Companies like Black Widow that make both recommend leaving your recurve (at least the one you're currently shooting) strung, but unstringing your longbow when you're not using it. I assume that's because there's a low risk in twisting the limbs of a longbow if you string it properly.
I would base my decision primarily on how well it shoots, although if it's been strung long enough to start losing weight, I wouldn't pay much for it, probably something under a $100.