Of course. There have been more sales than just the one you provided so I did a quick look through and found these. At a glance each looked like a valid sale, but I didn't price each one. I'd price each of them with overpay, etc and then resuggest if needed.

other methods include:

-going to previous suggestions

-asking friends with premium for owners bannable offense, if I'm not mistaken

Looking up owners with premium is something you'd only need to do with rarely traded items of which this is not. Usual practice is to look at the history of the hats currently on outpost/classifieds and that tells you the day someone was last seen with it. If the market is sparse old suggestions can also provide valuable links to hat histories which can point out newer sales. Once you have something that looks to be from the past month or two, go to that persons backpack and compare items that moved around the time the unusual you're pricing was last seen, be sure to double check the histories to make sure they match up. That's as simple as looking for both peoples names in the history. That's how I found the sale I gave you, it was an old owner of the Bonnet you sold. I saw it was last seen around the 25th of November in one owners backpack so I checked both around then to be sure and it was pretty clear that it sold for 13 keys. Outpost links can also be useful so its also a good idea to try and fish out a relevant one to the trade. Who knows, they might illuminate an aspect of the sale that was missed from the backpack scans.

Julia's script was a real game changer for us and it not only streamlined this process but made it much more reliable, definitely recommend you install them if you want to do more suggestions.

http://forums.backpack.tf/index.php?/topic/36100-useful-user-scripts-for-unusual-suggesters/

We usually only harp about finding more sales when there's only one sale in a suggestion and we know there's more. Hats like this always have more. If you have 2-3 sales then we're usually satisfied with that and we'll usually leave it there if nothing else is glaring at us. Since you have some links from Carnage this would be a good exercise to try and figure the sales out yourself and resuggest with them included. Odds are they're going to sit in the 13-15 key range and not much is really going to change, as tends to happen with cheaper hats like this, but they'll give a stronger representation of the market and that's always a good thing. Even sales with outdated hats can be useful as data points, just don't rely on them to set the range.

You already seem to have a reasonable handle on things so we're just trying to help you make this a better suggestion.