I would assume it depends on what's considered a normal form of transaction.
If any action is recognized by society as a form of transaction, halacha recognizes it -- this is known as kinyan situmta.
But Rambam and Shulchan Aruch rule that if you use "word alone" to commit to buying or selling (without any language of oaths), "though you have made no mark nor taken any collateral", it is "very appropriate that you keep your words; and if you don't do so, the Sages are upset with you."
So if IMing or emailing someone "I'll buy a dozen of your widgets" isn't recognized as a transaction, I don't think halacha would find it binding; but the concept of a strong religious value to honor your word would still hold.
Does that answer your question?