Um, may not be your fault -- they may not have been properly hardened off, could have been on a dark truck for several days, or kept on racks in the dark corner of the store, that sort of thing. They definitely need a lot of sun, etc. If the grower and/or vendor didn't treat them properly, as often happens with mass merchant nurseries, they could have been very "tender" and sensitive to the sun -- kind of like if you'd been in the north all winter, covered up, and then went to Florida and layed on the beach in full sun all day -- you'd fry to a crisp, too.

They may not be total goners, as long as the stem and the terminal bud are still green, they might come back.

However, if I were you, I'd take them back to HD and complain, make them give you some new ones, and put those outdoors carefully in the shade for a couple of days, then into part sun a few more days, then into full sun.

Fried cabbage is delicious. But you were a little early. Green house plants need to be hardened off before transplanting. Cabbage does like full sun, but any plant grown in a greenhouse is too tender to be planted under actual growing conditions, they need to be acclimated gently. Put them outside in semi shade for a couple of days and gradually introduce them to more sun and wind.

Thanks for the tip. I thought perhaps it was because of the row cover...my first time using this method of keeping those white "butterflies" off these plants.

No the row cover is quite breathable and would have provided a mild shade. I've had the same problem with cabbage transplants in the past. I bought from a local nursery who told me that they were hardened off....and had dead plants the next morning (no frost conditions either). That nursery provided replacements without a fuss. I suggest you call the HD and see what they will do for you.