@Happyladydiver hope you manage to get your Ebay claim sorted out.
Buying a "used" BCD is always difficult as it is hard to determine how well it hold air over time when pressurised at the time of purchase. This is especially true with auction sites such as Ebay where the sale is normally remote & dispatched.
Personally, while it sounds like a great system on paper, I am not a fan of the I3 system as it is a proprietary system, reportedly fairly easy to damage and difficult to service. A standard inflator is easier to maintain and repair (or replace) in the event of a fault or failure as well as being cheaper and more easily sourced. Also worth bearing in mind is ease of use in an emergency - everyone knows how a standard inflator works but might not with the I3. I should note that this is only my opinion and you can take it or leave it as you wish.