Question about Transmog

So I noticed the transmog scroll for legendary items and I'm wondering if you can Transmog the Tempest keep items at the end of the raid using those scrolls. You can pick those items up and they are on your character. If you get out of combat ( vanish invisible etc.) Could one transmog that item.

on a side note I would love to get Tier 3 Frostfire set from classic, but I know it's locked in this expansion. Would it help the economy to make a black market auction house with items that are unattainable for the purpose of transmog or rare drops like mana bombs recipe to be bought on such a place. This would reduce the need for frequent gold squish because people would invest more gold into t mogs at later game. Just an off color suggestion.

You can buy the frostfire t3 set on shop.

Where is the shop in Lordaeron that allows you to buy T3 set? I can get T7 which looks like the set bonus. Also still wondering if you can transmog gear at the end of tempest keep.

Don't think you can. Though, they're not entirely unique, they share models with other TBC loot, only the coloration is different.

You can add it as an idea.
open the coins store, at the bottom you have the "Item not in store? Let us know" button, click it and link the t3 gear.