Your "point 1" is literally the argument of the paper. If that scenario arises, the market collapses and no further sales can be made. That's the whole problem. As someone who takes a percentage of every sale, you want to keep the lemon-sellers out even though in the short run they make you extra money.
Your "point 2", if it's meant to be a rebuttal, isn't much of one. Buyers don't need to accurately know exactly what fraction of sellers are fraudulent; if they believe that it's 50-50, the same thing happens, even if the true rate is 80-20 in favor of good sellers. Conversely, if buyers are overly optimistic about quality, the market can persist despite a level of fraud that's higher than should be tolerated. But in any case, things like reviews and external reporting should eventually give them good information.
Your "point 1" is literally the argument of the paper. If that scenario arises, the market collapses and no further sales can be made. That's the whole problem. As someone who takes a percentage of every sale, you want to keep the lemon-sellers out even though in the short run they make you extra money.
Your "point 2", if it's meant to be a rebuttal, isn't much of one. Buyers don't need to accurately know exactly what fraction of sellers are fraudulent; if they believe that it's 50-50, the same thing happens, even if the true rate is 80-20 in favor of good sellers. Conversely, if buyers are overly optimistic about quality, the market can persist despite a level of fraud that's higher than should be tolerated. But in any case, things like reviews and external reporting should eventually give them good information.