You do not really ask a question here, but I am thinking maybe you are wondering if this is an odd event. If that is true, I will speak to that issue.
At age 6 kids are usually learning about money in school. So making "sales" might be just a normal learning experience, and if they are interested in money and how economy works, that's great. We all have to learn that.
However, perhaps you are concerned that your child is selling off the toys you have spent a lot of money yourself to purchase. If the deal was that the other child was to go home with the doll, and that is not okay with you, then you might explain to your daughter that playing store is different from selling her toys.
The giving away or selling of belongings is a grown up decision. I like to use the expression, "The grown ups are in charge of these things." (that leaves a lot of room for the "things."
If she is just playing "store" or "yard sale," or some such game, why not get some play money and let her learn. You can set a limit on what actually leaves the house in another person's possession.
Who knows, you might have a very successful business woman someday!