Rocketr's truly the only big alternative I can think of that's widely used besides Selly- great for the customer and very easy to handle as a seller as well.
Link to Rocketr here.I have heard
Sellfy mentioned before, but I have no experience using it as a seller or customer. I've mostly dealt with just Selly and Rocketr, but Sellfy's another one to look at for sure.
Forgive me, but i think that selly.gg is only created for scammers, most people in there has autobuys and they never give the item back.
This is true on Bitcointalk (especially threads with Selly links in Digital Goods), but on other forums I've bought plenty of products through autobuys like Selly and Rocketr, and I've hardly been scammed. It's unfair to assume OP's planning on scamming as well, but I do understand where you're coming from with the scammer statement.