He didn't have to. The offer didn't ask for people to reply to it. It was a statement that an action would be performed in the future conditional on others performing actions in the future. It's like, "If you bring us 100 pounds of corn, we'll give you $100 off on a car". You don't have to accept the offer, you just have to bring 100 pounds of corn to the place. Goat did what the offer required him to do -- he acquired shares of GLBSE. Nefario then reneged on his offer after Goat completed his part.

There doesn't need to be consideration in this case because there is detrimental reliance sufficient to establish promissory estoppel (see my other post). But there was consideration -- Goat acquired shares on GLBSE, transactions for which GLBSE receives a commission.