That is a very common objection. Here’s a video where I walk through how I handle it https://www.youtube.com/watch?v=d-pbzGGEp64&t=168s
In addition, the key things you are looking for are:
Are their listings well optimized?
Do they rank on page 1 for the keyword they are targeting in the title?
Does the product show up in the sponsored product ads for that keyword? (if not, this is a huge missed opportunity)
Are the current sellers replying to negative product reviews?
What you are looking for are reasons to show them that their current sellers aren’t doing a good enough job. You can want to find a way to get your foot in the door to show them you are better.
Perhaps they would agree to let you sell on of their less popular products so you can show what you can do?
Remember, this is all about demonstrating knowledge and building trust. That is not likely on the first interaction, so you must be persistent!
Thanks for posting your question