When a coin is in negotiation in a swap, I modify its public comment so it says "RESERVED". I believe many other members use a similar system.
However, these changes to the comments no longer show up in swaps that are already open. I work on swaps in the order that they were opened, in general, so it might be that a later swap partner has picked the same coin as an earlier one. When I edit the earlier swap and add the "RESERVED" comment, I want that comment to show in the later swap, so my swap partner can see that a coin is reserved and can pick another one. This used to work fine but recently (probably as of the change to the new UI) it does not.