This is the "end of the shell" that will tell you most of the story:
You'll see the indentation in the mantle. That's most likely a pinch. It's not a huge problem, and some report that a freshwater dip can fix it. I had a large squamosa that did that once, but it just went away. You'll find more examples if you did a Google on pinched mantles.
The thing that concerns me as much as gaping when choosing clams is when the shell protrudes further than the mantle and tissue (like in the picture). This should be at least even, even when the clam is closed. This tells me that the clam has lost tissue mass and may be sick, or was sick and is just recovering. If this is the clams full extension, it's not very healthy. If it does extend out more, chances are, you'll see some gaping, too.
I would ask for a lot more pictures before investing in this one.
Hope this helps.