I would implement product feeds from retailers (available from the big affiliate networks like Commission Junction, Linkshare, Share-a-Sale, etc). You would earn a commission on sales generated through these products. Since the model is about generating purchases, it seems to make perfect since to get a piece of that transaction. I am not sure, as a user, that I would pay to maintain a list because there are similar free options available on the web (thinking mainly about wedding registries like weddingchannel.com).

Get the .com if you want to have universal acceptance of the brand. A large percentage of the population assumes that every website is a .com, so if you stick with the .co.nz you will essentially be delivering free traffic to the gifty.com domain owner (since many people will just use the .com out of habit).

Its likely that the domain squatter will want more than you are willing to pay for the domain. You are best off just spending a few hours on the domain registrar of your choice trying to find an alternative name that is descriptive and available.