If you find, say, a nice weapon that you can't use but don't want to sell, what you do is go to the adventurer's guild, hire somebody, dump it in his/her/its backpack, and then dismiss the hireling and retrieve your own character. You can then get it back at any time, and this method is proof against respawn.
At some point, you will be offered use of some chests in the Merchants' Hall in Ravenshore in which to store goodies like these. Don't accept - the chests empty when the area respawns, and you'll lose everything you put in them.
Good luck, and enjoy the game. Have you already played MM6 & MM7?