Backstory: Weddings are expensive. The days of "my sweet 16" and overpriced weddings are over, people are becoming more frugal when it comes to this one-time event.
The Idea: A platform to resell wedding items.
Deep Dive: You would build a marketplace for users to post their used wedding items such as table cloths, vases, centerpieces, etc. Typically with two sided marketplaces you have to target one side first, this would be the wedding item provider. You could manually curate these from friends after their wedding.
Validation: This is a niche version of ebay such as Swappa for phones/electronics..
Monetization: You would take a percentage of the brokered sale.