You should first define the type of Multivendor app you're building. You can choose from B2B, B2C, and C2C models, depending on the category your idea falls in among these types. For example, a Multi Vendor Platform like eBay has a C2C model. You should do the due market research and get trustworthy and quality vendors onboard, depending on the demand of your market. Know your target audience, and provide a lucrative solution that can differentiate you from your competition.