People searching for "a panel with an API" usually mean one of two different things, and those two cost differently. The first is the inbound API: your reseller customers get a key and send orders to you programmatically, from their own panel or their own code. The second is the outbound API: your panel goes out to a provider, pulls the catalogue, places orders and polls statuses. Whoever is selling you "a panel with an API" usually means only the second one and says nothing about the first.
The difference is practical. The outbound API is what lets you trade at all: without it the catalogue has to be maintained by hand and orders forwarded to the provider manually. The inbound one is what lets you grow on somebody else's effort: a reseller plugged into you with a key brings volume you never had to chase. A panel with only an outbound API caps your turnover at whatever your own marketing brings in.
Our panel runs both, and neither needs custom work. You can connect as many providers as you like, each with your own key, and reseller keys are issued straight from the admin panel, as many as you need.