I was hired on to an Internet Provider to automate their billing system and ended up writing the dreaded eCommerce platform.
And in this case 'Platform' is the operative word; From the get-go the specs changed from week to week depending on what idiot the manager had talked to and told him what his application had to have and how it had to function in just specifically a certain way.
As the bank we had partnered with became more and more difficult to deal with the manager and accountant would think up more and more ridiculously obscure and abstract ways of accomplishing tasks that had become over-regulated.
The system ended up being big. Like, way big. Like, you don't really want to know how big. The only part of the software not written in perl was the code used to talk to the bank via a frame relay VPN for credit card transactions, and that was only because it was based on existing code (That didn't work).

• All shop owners would host their data on our site, but could link our site into theirs
• We had an internal mail system between shop owners and customers, including an address book and all
• Based on figures from various international courier/freighters the shop owner picked a freight company, and we worked out all the charges for them and contacted the freight company for item pick-up ourselves
• The entire system was insured, and as such had an Insurance module allowing representatives of the insurance company to review any insurances claims (Which had to be made internally) on bad sellers
• All sales history was naturally recorded, and there was a mountain of ways of displaying and graphing this data, thanks to the accounts two floors down
• Our partner bank decided not to allow us to pay money into a credit card, as such we would collect all of our shop owners profits during the day and then wire amounts to each country appropriately. Then we would manually dial in via modem to whatever chosen issuing bank we had talked to, and spread the money out to bank accounts from there, thusly avoiding a telegraphic transfer charge for every bank account we sent money to.

Those are the major features I can think of, off the top of my head. Most of those are still pretty frightening to me, particularly the insane method of paying into bank accounts... But there you go. Please laugh, it's all I can do now :P

JP,
-- Alexander Widdlemouse undid his bellybutton and his bum dropped off --