when a buyer logs in and presses the release funds for a purchase
is that instantly in my account if I'm a seller of the item they bought?
It depends on how the website is written. As it is closed source, we can't really tell. Except by asking people who have already made transactions. This may or may not reflect what happens for future transactions.
In any case, you have to be able to trust the website. If the operator of the website were, hypothetically speaking, stop the release funds options from working, he/she could then disappear with the funds. Especially easy to do when you are anonymous.
