There was a thread a while back that asked about a similar situation. I think the answers numbered into the hundreds.
The seller came back after a showing to find a coffee stain on a white carpet. No one assumed responsibility and I think that fact is what bothered people the most.
Answers were all over the board: most agents were aghast (I was one of them), some said assumptions were made that maybe shouldn't have been. The fact of the matter is that there was no proof of what happened and who knew what.
This is what I would do: assuming you have an agent, ask your agent to speak to the agent who showed the house. If you don't have an agent, it may be best for you to talk to the agent's broker. I don't know how much actual proof you have or what recourse you have if everyone denies any culpability but the more calm you remain in presenting your case, the more likely you can get some sort of resolution.
Obviously, if the buyers broke something, logic dictates that they should pay for it. My personal feeling is that if the buyers refuse, for whatever reason, the buyers' agent should be responsible because we are responsible for the people we bring into people's homes.
The biggest issue is if everyone denies knowing anything and you can't prove that this broke while the house was being shown , this may not be resolved the way you feel it should be.
I know this is frustrating. It shouldn't happen but obviously it does. Its possible that whoever was responsible will step up and do the right thing. If they don't, there may not be a lot you can do.