1. Have suppliers list their own items on my site (like amazon)
2. I list all the products and then I purchase directly from suppliers when I get an order?
Each of these two has pros and cons.
1) Why would suppliers take the time to do this unless you can guarantee them sales? If you go this route, your solution requires more complexity (UI for suppliers etc.) but means you will be more scalable.
20 This is a big burden on you but you are not relying on suppliers.
Is there a way you can automate the listing of product from integration with suppliers' systems?
Your customers don't care about your back-office processes - only what products you offer and your quality of service.
Also, how your products are listed on your site (manual entry from you, the supplier, auto entry by an integration etc.) is separate from how you make the purchase (indirectly as you suggest or directly from the supplier's systems.
Happy to have a call to discuss technical options which can then be considered and researched which best meet business requirements.