Me and my friend used to do this back when we didn't have any internet connection to download hacks :P
You need to have a character with the gold/item you want to dupe.
1. Go to your diablo 2 folder and the saved games directory
2. Copy the file with your character name
3. Paste it anywhere
4. Start up diablo 2 and start a server, your friend joins
5. You trade him the gold/item you want to dupe, he save & exits
6. You save & exit
7. You copy the file you pasted before and overwrite the file in the saved games directory
8. Now when you log in again you still have the item and your friend still has it.
May not be much, I'm pretty much a total noob on d2 :P
Thanks for reading & bye