Nintendo Switch Joy-Con (L) Neon Blue-Nintendo Switch

Same but it is   Only my left controller 

When I got mine it did not come with the slide with the strap on it

For a preowned joy con? I can't imagine why not. Just go when they aren't crazy busy and be polite and they should have no problem letting you snap it onto your switch to test. In case you didn't know, there is a place in the switch settings where you can test controller input so you can test every button for functionality. Even if you buy and something ends up being wrong after all, remember that there is a 7 day return policy on preowned for a full refund and 30 days for a defective exchange. Hope that helps :)