Summer sale cards that aren't marketable until after the sale is over?

It's not a bug, you can only use it for crafting another badge or it's a waste

Well, the summer sale cards are valid until 8/5/2017 so I'm sure you'll still be able to sell them til then, not 100% sure though as I've never sold any after the actual sale.

Nope, not a bug. One of the cards you used to craft your badge had a time restriction too so it passes over to the items that the badge generated.
The cards will be available to be sold/traded/turned to gems till the 5th of August though so you got time, even if they won't be dropping after the 5th of July

Aha, I see! Thanks, that clears it up then.
I thought they would be removed after the sale ended.