Since when did 13mm for a shaft become a "standard" or was considered "full size"? IMO its pretty stupid to order a custom cue with full size shafts knowing fully well that you're not going to play with them - but not getting them would affect the resale value.
As far as I'm concerned, a shaft no matter what diameter as long as it was turned by the cue maker is ok.
I hear what you're saying and on the playing part I agree that I can't play with a 13mm shaft, but trying to sell a high dollar cue with skinny shafts doesn't work well from my experience. I don't know where or how 13mm became the standard, but to get top dollar for a high dollar cue you have your best chance if you have 2 pristine 13mm shafts from the cue maker who built the cue.