What have you done wrong?

Well, your buyer had, I think, the right to expect the attachments that went with the machine – unless you specifically stated in the listing that the attachments which came with the machine were NOT included. 

A reasonable person would expect the machine to have all of the bits and pieces that it had originally in the absence of a clear statement to the contrary. The photo of the attachments that you did include doesn’t (in my view) convey that all of the other attachments were not included.

Trying to get what amounts to an extra transaction with that buyer is not on. Were you planning on giving eBay the FVF for the extra $60 you wanted? If not, that’s another issue (fee avoidance).

Were I you, I would send the attachments in a tracked satchel, apologise to the buyer, and cop the postage amount. Others on eBay may have a different view.

The above is purely my view as a buyer, observer, and offerer of opinions.