It must be a good chance to get free 500 EXP if anyone joins in the demo trading of this project.
It's like an Airdrop

When that Demo trade will be started and is there any additional rules to get involved in that Demo Trading...
The Demo trading has already started. There are rules set to this contest, you can check them out when you join
https://eosex.com/#/mock-trade Same as any contest, rules will always not allow cheating. Good luck.