The seller gets exactly the price on their contract and nothing more or less. The fee sits on top of it, so the buyer's cash is what funds it.
With round numbers: you sign the seller at $200,000. You assign your contract to a cash buyer for a $15,000 assignment fee. The buyer brings $215,000 plus their closing costs. The seller still walks with $200,000 minus their own costs. The seller is not paying you, though they may well feel like they did once they see what the house resold for.
On visibility, the fee normally appears as a line item on the settlement statement paying the assignor, which is you. Closing practice varies by state and by title company, and there are usually separate seller and buyer statements, so the seller may or may not be handed the page with your fee on it. The idea that a title company keeps the fee off the paperwork entirely is how people get in trouble. Your assignment agreement should state the fee in writing and both sides should have it.
One thing that shapes how big a fee a deal can carry: many hard money lenders won't finance an assignment fee, so your buyer has to bring it in cash on top of their down payment and rehab reserve. A $15,000 fee is real money out of their pocket before they've bought a single window. That's often the true ceiling on the number, more than what the house is worth.
