Questions

To Customer

Does the password need to be repeated on registration?
Are picture and list of interests required?
How does a user authenticate?
What are these restricted pages that require authentication?
How does a user login? Is that the same as authentication? Can everybody autheticate? Is this reset on logout?
Can authenticated users do the same as non authenticated ones plus more?
Does the old password need to be repeated in order to change it?
"As a user belonging to at least one organization, I can select which organization I'm currently working for." Shouldn't he belong to at least two? Or can he either work for himself or the organization?
Who is a seller? Everybody?
What happens with items that were bought? Does the seller keep it in his list???
Can I change the amount of items I have once one has been bought?
Can a seller remove items?
"the number of copies shown to other users must be consistent with ongoing transactions" What does that mean? Realtime updates?
"(temporal list)" ? What does it mean?
"As buyer or seller, I can watch a user, organization or item so that I can easily track activity related to them." What activity of organizations are you notified of? Can you filter this too?
should a user page show the user's credits or are these private?

[answered] How to deal with pictures? Allow only links to files uploaded elsewhere? Or find out how to transmit and store locally? Answer: see https://github.com/zombiecalypse/SinatraStory/blob/master/FileHandling.mdown