Here's a high level overview of what happens:
- You take a bunch of photos of your device, from all sides and angles, and post them along with a description and condition on Swappa's listing. You determine the price. Swappa does take a price-based cut, but for your Mac it would probably be something like $25 or $30.
- Someone might comment and make an offer, at which point you can either ignore them or lower your asking price to what they said and comment back saying that the list price has been lowered. Eventually someone buys.
- You wait until you have confirmed payment (!) and once payment has been processed (yes, through PayPal. They take 2.9% + $0.30 per transaction, I believe) you ship the device out and mark it as shipped.
Sellers define their own return policies on Swappa, so you do not have to accept returns for that 30 day window. I don't ever accept returns on anything I sell online. There's a little bit of trust baked into the process, but I usually document my packaging with a couple of photos before I ship it off just in case. I haven't had any issues