Perhaps selly isn't providing escrow, payment will go to the seller directly. So scam depend on seller.
As far as I know, selly.gg itself is not a scam. It's kinda like Western Union: they provide a great services, as long as you know who you're dealing with, and trust them completely.
And just like selly.gg, WU is often abused by scammers to scam naive people. But that doesn't make the service itself a scam.
Disclaimer: I've never used nor needed either one of these services.