Hi Parisa,
1) Some wholesalers may do this (indeed Sarah's co-op have enquired with a couple of suppliers whose prices were more than what they could buy at the local supermarket), but the majority offer significantly cheaper prices (we did a comparison and found co-ops save members up to 80% in NZ), especially since they can supply in bulk quantities instead of individually packaged items.
2) Cobuy does not facilitate the delivery, just the order process.
3) See above.

Great question. We are aware that there are several pieces of software in this space. I'd say we are the new kid on the block who are looking to do a mobile-first experience and that building software from scratch on new software paradigms and frameworks trumps old software any day. In other words "ones already existing for a long time" does not mean they're better - and can mean they're worse.

I do think our vision may differ - we don't want to just solve the admin problem of buying groups, we want to make the supply chain open and accessible. Getting there starts with co-ordinating  buying power.

Thanks for the question - that one has popped up before and I am still not sure if my answer is satisfactory. I will think about it some more.

Awesome thinking! Yes such things would definitely be where we wish to move in the future - a distributed protocol running on distributed infrastructure. The centralized management would occur by participants committing/allowing their nodes to be coagulated.

Please look at the awesome work done by the peeps on Secure Scuttlebutt - "Secure Scuttlebutt is a database protocol for unforgeable append-only message feeds."

https://ssbc.github.io/secure-scuttlebutt/
https://github.com/ssbc/secure-scuttlebutt

example apps being built on it:
Distributed Social Media: Patchwork
https://ssbc.github.io/patchwork/
https://github.com/ssbc/patchwork

Distributed soundcloud:
https://github.com/mmckegg/ferment

Git repos on SSB ;)
http://git-ssb.celehner.com/%25n92DiQh7ietE%2BR%2BX%2FI403LQoyf2DtR3WQfCkDKlheQU%3D.sha256
https://www.npmjs.com/package/ssb-git-repo

Big thanks to the ssb team - I cant wait to build on their amazing work.