“ Rabbi Shimon explains that anyone who so much as gathers these goods is unfit, but because of tax-collectors, everyone who gathers must 'sell' a part of the goods to the tax-collectors, and thus they are known as sellers of Sabbatical goods. ”

“ A gambler, especially a dice-player, in which the game is entirely concerned with luck, expects to win something though the odds are against him. Thus they cannot serve as a witness. Another possible explanation is that one whose primary profession concerns luck is not of solid character to serve as a witness in even a monetary case.
Loan-sharks - because lending money at interest is considered a sin (until later Talmudic reforms) , these people are not allowed to serve as witnesses for the same reasons as dice-players. ”