I have been given the opportunity to create and sell greeting cards. I would like some honest feed back about these before I begin working on the inside greetings.

very very nice!
if I should say a thing, just add a little bit more contrast ("body") to the pearls and to the lace, on the third one

The cards are looking good jeanniem.
Here's some worthy commercial printing info on setup and document area, bleeds and safety margins.
Two work flows to consider using guides; single face template with layers (easiest) and full template showing all sides.
I've attached a quick full template for perspective (US letter, guides might need adjustment).

If you use a single face template (w/layers & guides)
Gimp layer order (top down): Inside Right (rotate 180°), back, Inside Left (rotate 180°), Front.
Assemble with G'MIC, Arrays & tiles | Montage
Input / Output: All
Montage type: Auto
Merge mode: Scaled
[0] inside right, [1] back, [2] inside left, [3] front

Have fun!

Thank you all, your opinions and advice are most appreciated!

Very nice. They all have a happy, warm feeling.
I do have a technical quibble on the first, music themed card. The jagged edges in the musical notes and the curve jars. You've minimized the issue on the others. Perhaps work oversize, if you can, and reduce. Consider rendering from SVG or other vector format.

Take care,
Garry

I like those cards, they're cute. :3