...Mishna: And these are the ones ineligible -- to be judges or witnesses

one who plays with dice -- All of these people listed are thieves who will become rashas and therefore cannot be witnesses or judges

And merchants who trade in the produce of the Sabbatical Year - since lust after money is a sin from the Torah, he becomes like an evil, violent person, invalid to judge or bear witness, as he inclines after the ill-gotten gain (cf. Exodus 18:21)

when they have no trade but this -- people whose sole occupation is gambling are not involved in doing anything socially useful. As a result, they are unacquainted with basic business law and commerce, and have no aversion to illegal activity. However, one who engages also in some other form of occupation is not flawed in this way and remains eligible.

Gemara: Asmachta (literally "reliance") -- term used in the Talmud for an agreement based on speculation. In general, an asmachta involves one party (or both) consenting to surrender to the other a certain sum of money, depending on the performance or outcome of a particular event. In each case, the individual obligating himself enters into the agreement "relying" that the outcome will be favorable and that he will not have to pay. Since a transfer of ownership requires that the object's owner give it willingly, an asmachta agreement is non-binding, because the obligated party never sincerely meant to obligate himself.

it is non-binding -- Rami bar Chama considers gambling an example of asmachta, because each player consents to the terms of the game only because he expects to win. Since the loser does not willingly surrender the wagered amount, the winner is considered to be stealing when he collects, and is thus ineligible as a witness or judge.

any agreement such as this is not considered an asmachta -- a true case of asmachta involves a situation in which the man obligating himself has reason to believe that he will not actually have to pay. For example, one who contracts to work his neighbor's field and stipulates that if he does not live up to his agreement he will pay a fine, never really expects to pay, because he feels confident that he can do as promised. Another example: Reuven, a debtor, pays 50 zuz of the 100 that he owes to his creditor, Shimon. To assure Shimon that he will repay the remaining 50 zuz in a timely fashion, Reuven gives the note of indebtedness to athird party, Levi, and states that if he, Reuven, does not pay the balance by such-and-such date, Levi should return the note to Shimon, thus enabling him to collect the entire 100 zuz. Reuven does not really expect to have to pay the additional penalty, because he believes that he will be able to meet his obligation on time. Since both these cases are asmachta agreements, if the contractor is unable to work the field or the debtor is prevented from paying on time, he is not subject to the agreed-upon penalty. A wager, though, does not constitute an asmachta, because each player realizes that as skilled as he may be at the game, the outcome is nonetheless governed by chance. Therefore, he is resigned to pay if he loses, and the winner is thus not guilty of stealing.