Why? Everybody can buy, till the 80k Onions are fully sold.
You can make some restrictions, say only people who accepted for the airdrop can buy, max 1000 Onions per user.
This is not bad idea, but honestly, I don't believe that paying fee for exchange is a way to go for DeepOnion.
I'm sure they can arrange with some reputable exchange to list DeepOnion without paying or users voting. However, that issue if they will list it since most of the coins are still with devs is still questionable.