If I drop out many items at a place somewhere in the wild, will it disappear over time?
Some people report success with random stashes in the smaller maps (Kaer Morhen and White Orchard). There are some reported losses of loot stashes in Velen and Skellige.
Don't leave anything you cannot bear to lose. General 'stuff' that you haven't been able to sell yet should be fine for some hours on any map - I keep running into 'a small blackjack and a pair of boots' or 'two bear hides' or similar, as I pass back over ground... some of those were dumped 200+hours before as part of weight/value management.
You should assume that anything left can disappear at any time though.
Some merchants seem to be good at either keeping the loot items deposited with them, or providing upgraded examples of same... others discard their stock more frequently. The Fishmarket smith is quite reliable, as is the Putrid Alley merchant... but again... not a reliable and permanent storage.
If that is what you want, wait until 1.07 later this month.