So, I finally found the travelling merchant under Trading with your help, Angel, but there seems to be a bug about the quantity of the offered item. It said 10 Stone for 100 Silver, so I went ahead and confirmed it and got this message:

"You purchased 9 Stone"

I can't say if I actually got 10 or the 9 though, sorry. :s But either the text is off or the number is off with the offer the merchant had - or he just decided to con me and kept 1 stone. :]

Edit: I just checked again. Under Travelling Merchant I can still buy stones, it says now (because I did it 2 more times just to make sure xD):

7 Stone

Price: 100

Stone is a simple and cheap construction material.

If I click on "Buy" I get the text:

You purchased 6 Stone

and I have 100 silver less and 7 stones more.

Teilchen, thanks for reporting!

The bug is fixed.

Sorry for the inconvenience.

Oh, it's actually still there, worse this time though xD

One travelling merchant left my inn, a new one came in. Still offering me stones, this time 6 of them. When I bought the 6, I got this message:

You purchased 0 Stone

Sorry once more, this time it was a wrong message. Actually there were 10 Stones and if to judge by backend the trade was processed successfully though the displaying of it was wrong.

It should be fixed now. Please tell us if there will be another issues like this. Thanks again!