Publish a cck field value if other nodes contain a value in that field

Not sure if rules will be the solution to this but its where I've dived in, could be entity reference but here goes.

I have a site showing horses. Dobbin, Neddy etc with some information about them including videos. Users may have a number of nodes for each horse, eg, 20 x Dobbin nodes doing jumps on a different day

Users can optionally offer the horse for sale so there is a For Sale: yes / no field. If yes a for sale sign goes on the video. Simple enough.

The task is:

If user goes into a Dobbin node and selects yes to put the flag on for sale how can I automatically update it so that all Dobbin nodes have the same flag? Otherwise it involves going in to change all the nodes for Dobbin to yes. I need this to apply to existing & new nodes for Dobbin...ie if any Dobbin node has field value yes for sale then any Dobbin node will automatically have this for sale field value.

A secondary requirement is to limit this to the user, ie, I dont want any node Dobbin to update (more than 1 user could have a horse called Dobbin). I'm sure I'll be able to resolve this if I get put on the right track for issue 1.

Thanks for any help.

Jeremy