FFS, seriously. To complete a theft in this situation you must either commit a larceny, which OP manifestly has not done, or you must commit a fraud by false pretenses. However because OP swiped and the transaction simply failed, he has not committed that offense (theory of theft), either, because he did not have the requisite mental state at the time of the taking. ALL CRIMES (except for strict liability offenses not at issue here) require the union of act and intent. OP may have committed the act required (simply a taking), but he did not have the necessary mental state at the time of the taking and no offense occurred.

He likely has a contractual obligation to pay the seller and they can sue him.

Every single person in here claiming it is criminal theft is wrong and should be disbarred or fired.