Sales like this need to be taken on a case-by-case approach as this isn't a rule that always applies. These kinds of sales can be used within certain contexts, one of them being if the hat in question needs a lower range as established by other not-so-valid sales. And just by looking at the other sales, this seems to be the case:

-Sale 2: invalid quicksell for 375 keys, below current.

-Sale 4: bulk bp cashout, very likely to be below current

-Sale 5: Bulk sale for 800 Keys including a Misty Skull Warhood. From taking a quick look at the warhood, I found a single sale for 244 Keys: https://backpack.tf/profiles/76561198236425506#!/compare/1645920000/1646006400

That would put this sale at ~550 Keys(a more proper mini for the warhood would still be necessary), but this sale would obviously be below current.

Since pretty much every single sale(except maybe the rotation? though i couldn't find a good sale for that) seems to be against the idea that this should be 800 flat, there needs to be some sort of range, and that's where sale #1 comes in. If we were to place this suggestion in a bubble, there would be four options:

-Use sale #1 despite it being a resale for a conservative range (750-800)

-Use sale #5 for a wide range (~550-800)

-Use sale #3 to somehow give validity to the current suggested price(???-800, or 800-???)

-Leave unpriced since sales are all over the place.

With all of this in context, sale #1 needs to be used as it's the closest sale that could be considered as a "valid" sale, at least compared to the rest. If we were to still consider it unusable, that would mean this would need to be left outdated, as everything else in this suggestion points to 800 flat being too high.

Most of the time you will end up excluding these "buyer sells for less" sales, but you do still need to take them into consideration when making a suggestion instead of immediately discarding them. And for the context of this hat, I still think a range of 750-800 is reasonable.