Yeah... which is really bad for learning Polish :/ Well, at least we can make a different answer the main one.
Added "They bought".
Does this implying they were in the act of purchasing mushrooms, but did not complete it? We were buying mushrooms, then I saw an onion that I wanted to buy instead. We were buying mushrooms when I saw Sharron and started to talk to her. If I asked, "Why were you at the store?" wouldn't, "We were buying mushrooms," imply that the act of mushroom buying was completed and require a perfect verb?
That sounds like a good interpretation, also it can mean that they were buying mushrooms regularly.
There are some typos in there, but I don't have it in me to go to my computer and edit them out. I think my question is fairly clear, though.