Pressuring a Store to Take Back Purchase

I bought something in a store and wanted to return it. The customer service person was reluctant to take it back, but I was persistent and she eventually agreed to it. Was it permitted for me to handle it as I did, or did I violate lo tachmod (I heard in a shiur that you can violate this when pressuring a store to give a refund)?

Pressure to Include Second Storage Room

Buying a House with a Hidden Treasure

My student asked me the following. Shimon buys a house from Reuven at a normal price even though he knows (and Reuven does not) that there are many gold coins hidden in the attic walls. Is the sale valid? Why/Why not?