After you marry Lydia, she will open a new shop to make a small profit.

 * Bring her with you as your follower, make her wear another type of heavy armor, such as Ebony, then make her leave.
 * Immediately after that, see what she has for sale. She will still be wearing the Ebony and her default Steel Armor will be available for sale.
 * Buy all of it and keep it elsewhere. Sometimes she’ll have more than 3 items of similar armor.
 * After that, trade back the Ebony Armor from her, and she will be naked.  
 * Give her anything you want her to wear. (I gave her Vampire Lord Armor, and she looks smexy.) 

This works with console too, providing you have married Lydia.