What: A website that lets you shop cool and unique decor you see in hotels/restaurants/bars/high end vacation rentals worldwide.
1. Does something like this already exist?
2. Should it be an app or a website?
3. Should I have own e-commerce built in or just have outbound links?
4. What other steams of revenue can be included?
5. How do I test if there's a need for this?
I am not sure these are the first questions you should be asking. They have a number of positive assumptions built in to them which may lead to unsatisfactory answers. I suggest that you ask yourself what problem you are solving and for whom. Once you discover whom, I would interview at least 20 of them to start to see if the problem hypothesis you have crafted is important enough to them to want to engage with you. From those conversations you can learn a couple of things right away. 1. Are you solving a problem worth solving? 2. Are you solving it in a way that your customer values?
I also recommend that you look up Steve Blank, Eric Ries and Cindy Alvarez for more details on these processes.
you need a business plan. You will answer these questions and many more as you write one.
Doing it on enterprise-scale and I know others who's tech could be applied to the use case. There is a reason it doesn't really exist at scale. I know me and some others that are solving core infrastructure issues before we tackle this use case. If you can figure out where you're getting the data and how you are proving it was purchased or how to get paid go for the transaction; go for it. There is plenty of room for us all.