I noticed a customer did this today.
I charge roughly 90p more on ebay than I do on Amazon as the listing is set up on ebay to incorporate other products and I charge 1 fee for all of them whereas Amazon is listed individually.
I was not out of stock on either page and the buyer would be entitled to a nice discount had they bought the items together as they would have been posted together. Considering there is major competition on these products it appears that the customer has sought me out on both sites!
As it is I will need to post separately and provide two different tracking numbers.
I just think this is really weird!