How about to create a button on swap-page to automatically reduce number on coins by "-1" ?
For example, swappers are both confirmed a swap. So, it means (in most cases) that both send negotiated coins to partner, so it's number will decrease by "-1". It needs to reduce every coin in a manual matter so it's a source of errors (miss to reduce, twice reduce and other). So, exchange banks are became unrepresentative.
So, such button can auto-reduce every coin which are exchanged. User can use it or can do it manually, as former. And, maybe a warning must be displayed on swap-page and in swaps-list something like that "Afterswap number of coins is not reduced" - also it must be an way to change this status manually.
I'm sure that it will be very useful. If so many people are asking for that - it's time to think about this!
For me, Numista is very useful exactly about swaps. I searched for site with auditory and tools for swap and happy that I finally found Numista. But it's always have a directions which can be enhanced.
In the meantime you can use the "edit my collection" button within the swap page which opens up all the coins so you can edit them all easily, but it would be nice for it to be done automatically once the swap was confirmed.
I would love to use that, because sometimes I am swapping with 100+ coins and after that I have to spend too much time to clean comment fields (reserved for...) and change number of coins and it is very easy to make mistake- I did few times and it is annoying to say next swapper: sorry, your selected coin actually is not available anymore. also I am pretty sure that I still have some coins what was deleted by mistake from list.
But I trust it would become handy to a lot of us!