Took me a long time to read through all of this - very interesting stuff.
I'm not an active seller but I do take an interest in eBay's injustice system. I've sold a few things recently and even that was a pain.
Here's my thoughts. These are only suggestions and by no means represents any action taken by me, or recommended to any other parties.
a) make their system work against them. Set up some buyer accounts, buy low cost **bleep** from china with postage included etc. When it's marked posted, leave bad feedback and open up disputes etc. Flog the **bleep** out of the system and start getting those seller accounts shutdown. Let them take on eBay and eBay can then determine if their system is working or not.
b) start a cooperative selling platform of your own. Run it along side your eBay business for starters and ditch eBay later on. It sounds like you all know each other well enough and probably know more again. A lot of work, true. But look at gumtree .. it was **bleep** in the beginning, and still is in my book (eBay is no better .. bad interfacing, bad systems, bad support, bad policies etc) but it had humble beginnings. This does interest me from a tech point of view.
eBay is a classic example of a system that has been patched and upgraded over the years to where it is now. A fkn huge mess. It is, without a doubt, DATED. Imagine a retail store (chain or local) that had systems like this.
Anyway .. it's an interesting fight. I hope some good comes out of it.
