I know only one example of the opportunity to buy tokens on the exchange before the end of ICO. But the trick was that you could buy tokens; sell opportunity was not
.
In this trick, except for listing there is nothing. I have seen projects that have done and are doing this now, and this does not bring them anything extraordinary.
Well, if team distribute tokens in an agreement with the exchange, it is quite possible that until the end of the ICO they can make the listing. But users should not sell until the end of the sale. This can be done with a smart contract.
That's the way it works. In this case, you can only buy. And I repeat that there is nothing here but an actual listing, which is generally not bad. At least this will add confidence in liquidity in the future.
I think the project should successfully complete the ICO, only after that the team can make a listing.