What's the one sentence explanation of why Onista is better?
"Onista is an auction-less social marketplace where people can buy and sell products." doesn't tell me why I should care.
1. The one liner I suggest forgetting about marketability, memorability, etc. of the wording, and trying to continue this conversation with a long time ebayer:
Onista is an auction-less social marketplace where people can buy and sell products."
"Huh?"
Use as many sentences as you need. Work out the meaning, then get it into a zinger. If you can't well then find another way of communicating that. Not everything fits into a one-liner. There are other ways of saying things. Start by working out what it is people need to understand before giving it a try.
2. the terminology