Thing is, TF2 is kinda dying. When you don't play it competitively it does get unexciting rather quickly, and the effects can be seen in both playercount and trading. Most people I know have quit TF2 to play other games (mainly CSGO), and the majority of them have sold their TF2 valuables, or even their entire backpack.
(as a side note, everyone here with expensive items should probably do the same since TF2 in general, specially the economy have only been going downhill for over a year)