LCD broken, or something else?

Yes most probably an LCD. But you will have to confirm it by testing with a known good LCD.

If I were you, I'd stock 2 LCDs. When/if I sell the 1st LCD, I'd immediately order a new one. This way you'd always have at least 1 in stock in case a repair comes up, or you need it for diagnostic. Time is money: if you're shopping in a hurry, you won't get the best deal. If you're ordering parts to stock them, and don't have a client impatiently waiting for that part, you'll be able to trade longer lead times for better prices.

And those LCDs work for both iPad 3 and iPad 4.