how do you change (in the forum exchange) your shop from auctioning to selling? Or do you need to post another thread?

No, do not post another thread. You're only allowed one thread.

How you change it is... Hit edit on the first post, click go advanced. You should see the drop down beside the title to select selling.

You would click "Edit" on the first post>"Go Advanced">Go to the "Title:">Drop down that menu and click "auctioning". There you have it!
You would not make another thread in The Mene Exchange, every user is only permitted one thread in that forum per account, when you are down using it change it to "nothing ATM" to keep it your stationary forum. I hope I helped!