You can have us insert your FB tracking pixel code in thank-you/download pages we generate for your buyers. To have us insert the same code for all orders regardless of the item(s) ordered, paste that into Seller Admin > Edit Preferences > Common Thank-you Page HTML. To have us only insert the code for sales of a specific product, or insert different code for different products, paste that into the Product Thank-you Page HTML field of that product's settings.
Also bear in mind the additional tracking tips we provide here: