It may repair itself, but a newbie is not necessarily going to be the best rehab agent, and you could both come to grief. The second one sounds in worse shape. I think you should avoid this store.
Start your reefing career with livestock in prime condition. If you're in a place that doesn't have a good lfs [and the clam tank you describe gives me pause] try dealing online with one of our sponsors. The one I personally have dealt with is Live Aquaria, which is connected to Foster & Smith, an old and reputable company...they have an 'arrive alive, stay alive' guarantee, and if you got a clam from them, I feel you'd have a better start.
Read up very thoroughly on clams before you get one. Their internal anatomy is something out of Star Trek. The croceas want, for instance, mh lighting, and they all take feeding with phytoplankton [again, you can buy it locally or off the net]. They eat only that, and are fussy about placement, but otherwise a real pleasure. Getting one will likely put a big hit on your tank calcium, once it settles in and starts shell-building: be ready for that. And they like very good water: I keep corals, with params like this:
alk: 8.3
calcium: 400-420
light mh
temp 80
salinity 1.025, and steady.
I didn't start out to keep clams, but the one I have is happy as...
Hth. I do wish you all kinds of luck.
