This is going to vary from state to state. Potentially you could be considered a money services business, a money transmitter, or you may need to become licensed as an escrow service. You'll want to look into Fincen's guidance on whether you are a money transmitter. You'll also want to look into your state as well as other states money transmission laws. Typically speaking as soon as your holding funds for others or transmitting money you'll need some type of registration or licensing. MSB licensing can be incredibly expensive and a very long process. If there's any way to avoid holding funds I would do it for simplicity as well as to avoid liability.
I've been lending capital, consulting and raising capital in the B2C lending space since 1998; generally AFS [Alternative Financial Services - both collateralized & noncollateralized. [Think payday loans, car title loans, installment loans...] Started with a brick-n-mortar and transitioned to online. If you're planning to lend, you face a multitude of regulatory hurdles and compliance challenges on both the federal and state/province level.
If you're "holding" funds, you face the same challenges. Google "Fintech - fill in the blank" and you will begin to comprehend the reg issues.
What "service" are you offering? Take a look at Dave.com for some BG on what I'm referring to.
You have not offered enough detail to dive deep into your question.
Jer Trihouse Consulting
These services are great for online transactions. These technology companies are, normally, self-regulated and self-governed. This can be seen in this link https://squareup.com/us/en/legal/general/ua for Square.
They are tech companies that help transact business. Their security is through their own mechanisms. Banks are FDIC insured.
Hope this helps!