Ask HN: Marketplace transaction processing - server side or client side?

We want to create an online marketplace but none of us had any experience with handling transactions. We are looking to integrate 3rd party tools to do that but wondering what the pros/cons of implementing the calls on the client side (Javascript) vs server side (C#).

The choice of implementation will also influence the 3rd party framework we choose. Brain Tree offers a C# API (I'm more comfortable with the server side, being mostly a desktop developer). On the other hand, we have been highly recommended Stripe (they don't have C# API) but I don't have enough experience to decide whether using Javascript to submit transactions is secure enough and/or proper implementation (we'll need to store some transaction details in our own DB too).

Any thoughts, suggestions, recommendations of other transaction processing frameworks are welcome!