Want to build a marketplace for selling baked goods. Roast my idea.

I could see my wife and I using something like this for holiday's and parties. Esp if A: We could trust the ingredients wouldn't kill anyone (i.e. nuts) and B: knowing we where supporting local/ home bakers.
A couple other thoughts:
Who are you targeting? Retail or wholesale or both?
There are some bakeries in our town (northeast) that have a store front and also sell wholesale to like restaurants and gas stations.
Maybe partner with a existing bakery or 2 to start?
Additionally you could look into setting up service with grubhub or door dash (maybe I know nothing about working with them) But you would have some important components taking care of (production and delivery) and you can focus on the marketplace itself.
Final thought: You could promote with eating contests. Who doesn't love a eating contest?

Interesting question, I know a buddy working on the same. If I remember correctly it's called www.BakersMakers.com.

How do you ensure and enforce food safety and quality?

Markeplaces is always a chicken and egg question. You have to promote both sides) how are you planning to do so? What is the market size in your location for online sales vs offline? I would suggest (as another idea) you to sell with discount (40-70%) unsold items after 6pm so restaurants and bakeries no need to throw it away. I am now helping one startup on that area with marketing, they just closed their seed round.

Like etsy for cakes? I like this idea. But off the top of my head:

• I imagine food is hard inventory to manage. Your marketplace will only be as good as your bakers, so there will need to be some kind of manual vetting process, at least in the beginning.
• How would you monetize it?
• How do you find and convince bakers to sell here?
• How do you find people trying to buy baked goods online and convince them to use this?
• How will you handle disputes between buyers and bakers? Even if you assume no liability, you have a responsibility to remove bad actors from the platform. Otherwise no one will trust you. How do you determine if someone acted in bad faith?

How can it beat Facebook marketplace?

Sounds good but I would definitely look into what you are allowed to sell and where you can send it to.

I am in the UK and the Food Standard Agency have a website detailing it

https://www.food.gov.uk/business-guidance/distance-selling-mail-order-and-delivery

I am sure you will have something similar where you are located