Will Steam Summer Sale Cards be able to be sold after the sales finish?

Cards will vanish after 2017-8-5 18:00:00.

Your title and your question are in conflict.
The sales have already finished. You can still sell the cards until the expiry date.
But you can't sell the cards after the expiry date which is when the cards disappear.

thanks!

If I recall correctly during the last Winter Sale once the sale had finished you could still list the cards on the market and trade them up until the cards expired.

My notes from the last Winter Sale say "Cards were still tradable on the market after the sale ended", but I think I meant to say "Sellable", I could be wrong though so don't take my word for it.

yes you can list/sell your cards from now or after the sale for 1 month in the market