Hey Art!
It sounds like you are fairly well convinced that this is a 11a and not a 10a and if the color is anything close to being real I would agree with you. I congratulate you in seeking the input of others before hammering a
ebay seller. I am sure you realize, leaving a item not as described negative feed back on
ebay is a disaster for sellers.
I might look at the overall seller prior to leaving any feed back. You said he was willing to work with you and take the item back and issue a full refund, including shipping? What does the rest of his feed back look like? Is there a trend of other buyers having issues? What were the other items he had listed? Is the seller listing the normal
ebay fodder or this an upscale seller? Also how fast did he respond to you?
I think all these need to be considered, after all anyone can have a bad hair day, well maybe not me. Most of mine fell out several years ago. LOL

Ron