I have a hard time with the scale of this quilt, so am not able to tell how large it is. Carpenters Wheels generally use pretty large pieces, so it could be anything from a baby quilt to a full sized bed quilt.
Seeing that you've got a buyer for the quilt, though, I don't think anything less than a perfect finish will work. That means that, in addition to taking the quilting out to remove and correct the pieces that are wrong, the quilting will have to be duplicated exactly.
If it's a small quilt, you could trace the quilting of the area that needs to be fixed, plus a little further, on a piece of tracing paper; shouldn't be too hard. Then carefully pick out the quilting to just a bit past the pieces that are wrong. Get them corrected, lay the tracing paper over and very, very carefully quilt over the lines you traced.
If it's a large quilt and I were keeping it, I'd still try that method. However, seeing that it's sold, I'd remove the quilting, fix the piecing and ask the quilter about quilting that area again.
It's too bad the quilter didn't notice it, but if it's loaded on a LA and then being rolled along, it's possible that without seeing the whole design she or he didn't even see it.