Could a left hand shake seal a bet?

Yes. Although a right-hand shake is the custom, both honest people and dishonest people pay little regard to which hand is used to 'seal' the bet. An honest person intends to honor the agreement, and a dishonest person intends to welch-- whichever way they can-- regardless of which hand was used for the 'shake'. Recent historical research has shown that no honest person over the age of 12 years old has ever uttered the phrase, "...it doesn't count because I used my left hand! ha ha ha..." That research had an error rate of 3.7 percent. Additionally, through birth anomolies, accident, punishment, or war, a number of people either don't have a right hand, or their right hand doesn't work properly. Those people are allowed to 'seal' bets with either their left hand -- or as the Christian Bible commands-- with their Word only. (Matthew 5:37, James 5:12)