Can a sell form seller create an account later?

On Pro and Enterprise, yes. A seller who submitted through the sell form can create a Resale App account with the same email address, and their earlier submissions are attached to it. They need one to accept an offer, and it also lets them see their offers and payouts in one place.

On Launch there is no Resale App, so sellers don't need an account: they answer offers and add payout details from the links in your emails.