It's a to b, not a then a1, a2, a3, a4, 72, Square, Pickle, B.

Sale 1

http://www.tf2outpost.com/trade/24922254 Sold for 35 keys

X = 2.8

Keys https://backpack.tf/profiles/76561198016238290?time=1422864000#page1

Sole https://backpack.tf/item/3390865016

Sale 2 OUTLIER, LOOK BELOW 3rd SALE FOR REASON

http://www.tf2outpost.com/trade/24674832 Sold for Steaming Metal Modest

B/O for Modest was 2.2

Sole had no trade, presuming offered

X = 2.1

Modest https://backpack.tf/item/1655121393

Sole https://backpack.tf/item/3390865016

MiniSale 1 for Modest

http://www.tf2outpost.com/trade/24570664 Sold with 5 keys for Twatt Gilded

B/O for Modest was 30 keys (2)

X + 0.3 = 2.3

X = 2

Gilded https://backpack.tf/item/3350181285

Modest https://backpack.tf/item/1655121393

Mini-Sale 2

http://www.tf2outpost.com/trade/24730866 Sold for DP Virus Doctor + 10 keys

X = 1.4*0.9 + 0.8

X = 2.2

Doctor https://backpack.tf/item/2948445532

Keys https://backpack.tf/profiles/76561198084633786?time=1422345600#page11

Modest https://backpack.tf/item/3074376808

Using 2.1

Sale 3

http://www.tf2outpost.com/trade/24780260 Sold for Aussie Medigun

Medigun had no trade, presuming offered

B/O for Sole was 2.7

X = 2.7*0.9

X = 2.4

Medi https://backpack.tf/item/3296716307

Sole https://backpack.tf/item/3191817122

Sale 2 presumed outlier due to Sale 1 selling for 2.8 buds in 3 days, and Sale 3 selling for an Aussie Medi-gun in 2 days.