Do I need a money transmitter license if my platform collects customer payments and distributes proceeds to vendors?

The answer vastly depends on what relationship you have with both the customer and the supplier.

Let’s start with the Customer. What would the customer see as the name of the business on your invoice? Would it be yours or that of the supplier?

If the name is yours, then essentially, you are selling the goods directly to the buyer. In which, you’re nothing more than a trader. You ought to be fine. Also, provided you can show a dotted line agreement to the supplier (i.e. your vendors). The goal here is that you must accept payments into your own account and then you must payout to the vendors directly. The vendor cannot accept payment directly, and the MOR (Merchant on Record) is your company.

Vendor. If the vendors can accept money directly or Through your account, but you are not the merchant on record, then you are an Agent and this might require a money transmitter license. There are certain states that allow you to act as an Agent of the Payee and this is called Agent of Payee Exemption from money transmitter licenses. At last count, I think there were about 12 states that provided such an exemption, but even that is diminishing.

IANAL standard disclaimer.