because, unfortunately, the law prohibits nonFFLs from having access to NICS.

Sounds to me like a person bought the firearms legally, then sold them to Cross.
Did he know that he, Cross, was a felon?

Not sure how this one is the store's fault. Wlmart is required to run a NICS check on all sales. If the buyer didn't come up as a felon, that's on the feds, not the store.

They have the forms on file. All they have to do is pull them out.

Were both present at the sale? If not how whould the store kniw? They performed a background check and the buyer apparently passed.

Walmart, Friendly Firearms, etc, are almost certainly going to prevail, if they conducted the proper background checks. So whoever is bringing the suit may be responsible for the legal costs of all.

The gun buyer, Reidle, may be a straw-purchaser, but it's not easy to prove, and he probably doesn't have deep pockets.

Good luck.

...a willig party to a straw purchase which can be accomplished without any real third-party conspiracy.

In general, the Big Box stores have standardized training and a set of corporate ground rules for the clerks. There's not a lot of room for individual decision making. Plus, it's easier to get away with a straw purchase simply because the clerk can't always see the other person in the crowd or in a big store. e.g. "I'll tell you what I want, then stand over here in camping or the fishing department while you pay for it." kind of thing.

All the LGS I go to are smaller family businesses and I've personally seen them deny a sale because something just didn't feel or look right.

YMMV

You know how much they loves them some loopholes.

Or at least claim they found one and score some kind of "win".