And what about the practitioner one?

As a side note: when you exclude something that resells for (much) less, you're implying that what they paid is too high and what they sold for is too low. In this case, it would imply 13 is too high (which it most definitely isnt). While they may still not be the most reliable sales, it poses a problem when it is used to exclude all low sales this way to only be left with the highest ones. Actually, in this particular scenario, 17 sold for 16, 16 sold for ~13, which resold for 13, which resold for 11, so then all sales would be invalid.

Be aware that the selling-for-less clause is usually meant to avoid suggestions with people paying grotesque amounts for hats they then end up dumping. The larger the difference, the more likely it is that the sales are outliers. This works best if both sales are pure, or with hats where it is very obvious that the person paid way more than they resold for. If someone pays ~14 in hats and then resells for 13 keys, it may just mean he wanted 13 keys to begin with, in which case its fine to use. Same when someone buys for 20 in hats and then resells for 19 in hats; the personal difference in what people value those hats at may easily explain why they made a deal that seems slightly unfavorable on paper, and if it's this close together, then even the range(s) on the hat(s) could account for the difference.

In short - try to use common sense to determine whether or not a selling-for-less case should be excluded, in particular try to avoid lopsided ranges as a result of it.