Re-suggesting, looks like 17.77 ref for low-end would be better now. I'm closing the previous suggestion too.

Classifieds:

http://i.imgur.com/pvf7WZS.png

Sellers for 17.77 ref and under for first 3 pages. Sellers for 17.88 ref and under for the next 2 pages, so 17.88 ref sellers and under for the first 5 pages.

First 3 pages range from 17 ref to 17.77 ref for buyers.

Scrap.tf:

https://scrap.tf/keys selling for 17.77, buying for 17.55 ref.

52 keys available

http://i.imgur.com/YXVK9Nx.png

Trade.tf

http://www.trade.tf/mybots/sellers/Mann%20Co.%20Supply%20Crate%20Key

4 sellers at 17.77 ref and 5 sellers at 17.88 ref.

http://i.imgur.com/532EBJ1.png

Outliers:

http://www.tf2outpost.com/trade/28052606 16 ref

http://www.tf2outpost.com/trade/28045749 17 ref

http://www.tf2outpost.com/trade/28028452 17 ref

http://www.tf2outpost.com/trade/28047281 17.66 ref

http://www.tf2outpost.com/trade/27861411 17.66 ref

http://www.tf2outpost.com/trade/28043618 17.66 ref (low end)

http://www.tf2outpost.com/trade/28041606 17.66 ref

http://www.tf2outpost.com/trade/27745684 17.66 ref

http://www.tf2outpost.com/trade/28033884 17.66 ref

http://www.tf2outpost.com/trade/28030896 17.66 ref

http://www.tf2outpost.com/trade/27428948 17.66 ref (previously 18 ref seller)

http://www.tf2outpost.com/trade/28051961 17.66 ref

http://www.tf2outpost.com/trade/28054727 17.66 ref

http://www.tf2outpost.com/trade/28053025 17.66 ref (bulk)

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http://www.tf2outpost.com/trade/28007676 18.11 ref

http://www.tf2outpost.com/trade/25718541 18.33 ref

http://www.tf2outpost.com/trade/27745231 18.33 ref

http://www.tf2outpost.com/trade/27941503 18.33 ref

http://www.tf2outpost.com/trade/27767886 18.33 ref

http://www.tf2outpost.com/trade/27277215 18.55 ref

http://www.tf2outpost.com/trade/27843086 19 ref

http://www.tf2outpost.com/trade/25784314 19 ref

http://www.tf2outpost.com/trade/28025835 19.77 ref

Sellers:

http://www.tf2outpost.com/trade/28046177 17.77 ref (2 days)

http://www.tf2outpost.com/trade/28046986 17.77 ref

http://www.tf2outpost.com/trade/28032668 17.77 ref SOLD

http://www.tf2outpost.com/trade/28036609 17.77 ref

http://www.tf2outpost.com/trade/28012346 17.77 ref SOLD

http://www.tf2outpost.com/trade/27969162 17.77 ref (previously 17.88 ref seller)

http://www.tf2outpost.com/trade/27791070 17.77 ref

http://www.tf2outpost.com/trade/27710333 17.77 ref

http://www.tf2outpost.com/trade/28053668 17.77 ref

http://www.tf2outpost.com/trade/27916292 17.88 ref

http://www.tf2outpost.com/trade/28043785 17.88 ref SOLD

http://www.tf2outpost.com/trade/27697944 17.88 ref

http://www.tf2outpost.com/trade/27348151 17.88 ref

http://www.tf2outpost.com/trade/28046178 17.88 ref

http://www.tf2outpost.com/trade/28044840 17.88 ref (3 days)

http://www.tf2outpost.com/trade/28041487 17.88 ref (4 days)

http://www.tf2outpost.com/trade/28040929 17.88 ref (1 day)

http://www.tf2outpost.com/trade/27952588 17.88 ref SOLD

http://www.tf2outpost.com/trade/28035689 17.88 ref (5 days)

http://www.tf2outpost.com/trade/28025561 17.88 ref (6 days)

http://www.tf2outpost.com/trade/28045645 17.88 ref

http://www.tf2outpost.com/trade/27546363 17.88 ref (previously 18 ref seller)

http://www.tf2outpost.com/trade/26910525 17.88 ref

http://www.tf2outpost.com/trade/25463293 17.88 ref

http://www.tf2outpost.com/trade/28050070 17.88 ref

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http://www.tf2outpost.com/trade/27674070 18 ref

http://www.tf2outpost.com/trade/27355325 18 ref

http://www.tf2outpost.com/trade/28047037 18 ref

http://www.tf2outpost.com/trade/28046833 18 ref

http://www.tf2outpost.com/trade/28011857 18 ref

http://www.tf2outpost.com/trade/28029498 18 ref (4 days)

http://www.tf2outpost.com/trade/28022220 18 ref (5 days)

http://www.tf2outpost.com/trade/28001121 18 ref (1 day)

http://www.tf2outpost.com/trade/28042673 18 ref (3 days)

http://www.tf2outpost.com/trade/28028286 18 ref (1 day)

http://www.tf2outpost.com/trade/28037191 18 ref SOLD

http://www.tf2outpost.com/trade/27977060 18 ref

http://www.tf2outpost.com/trade/28053659 18 ref

So now the keys for 17.77 ref are being even more common and probably will increase after this one. I will try to update with more links of sellers at 17.77 ref if possible. Keys for 17.77 ref and under aren't hard to find anymore either, which probably is why some 18 ref sellers don't have their trades bumped. Some 18 ref sellers have lowered their price too, so that makes it even easier to find keys for cheaper. 18 ref will still be in the range since there are still some keys being sold for 18 ref. Also 52 keys have been sold to scrap.tf for 17.55 ref... So some are willing to sell theirs for a lot lower than 18 ref, which is why I'm suggesting 17.77 ref to 18 ref. The 17.66 ref sellers are increasing too, so that may be something else worth noting.

You can also easily find sellers at 17.77 ref at classifieds now, which is one of the reasons why I'm suggesting 17.77 ref to be low end instead of 17.88 ref.