first back up your chestfile,then put any items you want to trade in the shared stash tab,go to where your saved games/chest file is
and send it as an attachment via email,once you get chest file sent to you put it in with save games and load up your toon and the items you
received are in shared stash tab so put on toon and if want put back your own chest file and thats it. allways back up your own chests.
this works for sp and doesn't effect your toon in anyway, matter of level/where you are, so dont worry.
just make sure you dont have game running when swapping chests
My method is a bit more involved cos I have learnt from D2 trading days
First rule of a Single Player backup your game save file lol
I create a trader character say SW, do the dash to Sloeford
F9 save exit
Open up the character with the items for trade, place them in the shared belongings chest.
F9 save exit
I then winzip/rar the chest file and place in a folder called outgoing saves(I have an incoming one as well).
With an item/s I receive I take the chest file open it and place items in my characters inventory then F9 save exit.
I then cut/paste my backup save chest file back in original saves( I keep all my character gear on their side the shared side is generally empty).
Then open my trader character and put items in shared belongings for which ever character needs them.
I find if your in doubt of anything in SP create a backup, saves a few tears and stressful moments