Hey, I think the number one issue you're going to face is a factor of viability, to them you're a new website, not really well known and even if they give you the listing of the products, will you actually be able to generate interest and sales for them?
Once we understand this it would be much easier to tackle them in the following way
1. Ensure them a budget from your end on digital spend which will drive variable X amount of demand
2. They have nothing to lose, you aren't physically taking possession of their product unless it is actually rented out
3. To put it on the website you will need highly clarity images, which you could agree to share with them for their own social media
4. Use a couple of stock images, run some test campaigns and some them a basic real time demand on paper which would leave them calculating the potential revenue in their own head
These are just a few things, hope it helps!