We have ways of making things easier when it comes to cleaning decanters!

Try giving it a good soak in lukewarm water with biological washing liquid - the really violent, skin-destroying enzymatic stuff. It will eat up anything biological from the inside, which can then be dislodged with a shake with some dry rice or, if you have them, "magic balls" - copper ball bearings.
To make sure it really is dry afterwards, pop a "wick" of rolled up kitchen roll in it and leave it at least overnight. Any water vapour coming off will absorb into the towel.
I have bought a lot of good glass in TKMaxx - I don't see as much of the top quality stuff as I used to though. I still keep a good eye on what is there now - you see a lot of it on "antique" stands, being attributed to being something other than what it is!