Sacred 1 has ton of classes and you are bound to find sets, uniques or other very good gear for the classes you are not playing at the moment. And while there is no shared stash in the first game, why should these items go to waste if you ever decide to roll the class later down the road? Fortunately it can still be done but requires a lot of steps. Just managed to successfully do it by following a guide on the sacred wiki, but it omits a few steps. I will lay down what I did to successfully make it work. Before you do anything, first BACKUP your saved games in case something goes wrong before you start doing this. They are in the game install location/save.

1) Load the game with the character that has the items you want to transfer.
2) Export it through the menu into an empty slot.
3) Go to Multiplayer - Local Network, select the character you exported and click Next.
4) Click Create and change game type from Hack & Slash to Cooperative. Difficulty should not matter, but better leave it at bronze or silver so all your future chars can access it.
5) Once the game is started, drop the items you want to transfer. If you are doing a bulk transfer of various items for various classes, drop them together in a pile so you can easily find them later.
6) Once the items are dropped, Save the game to a new slot. Name it whatever, like "Item Transfer" or something, doesn't really matter. Then quit the game.
7) Go into Multiplayer - Local Network again and this time, create a character you want to play next. Once you get to the game setup screen, select cooperative again but this time, don't click create but Load Game and load the game you saved with the first character.
8) Pick up the items you want to get. Once you have done so, just quit the game without saving.
9) To start this character in single player, go to a Campaign and import the hero that just picked the items up.
10) Congrats, you have started a single player game with the transferred items.

A couple of notes:
1) Some or all of the items may fail to show up after loading the game. If that happens, quit and load the game again, generally, all items appear correctly for me after the 2nd load. Really strange behavior but no idea what to do about it. The items are still saved, just fail to appear sometimes...
2) Whenever you want to add items to the LAN game save file, just enter it again with your exported single player character, drop the new items and save over the original file.
3) As this method relies on character exports, it is only feasible to transfer the items when starting a new character, or moving up to a new difficulty. There is no way to transfer items to a single player character mid-difficulty without starting a new campaign.
4) The items will stay on your original single player character (single player and multiplayer chars are separate) but as they are now backed up in the saved LAN game, you can just drop/sell them (unless you want to dupe items of course).
5) Assuming you are playing your first char, you can create and save the multiplayer game (steps 1-6) once your chest gets full and then later down the line, any character can drop off additional items in that save game. How you go about it depends on you (e.g. creating a separate LAN game save file for items for all the classes or just keeping one big one). I recommend keeping one for each class, because when there are a lot of items, it can get quite messy navigating in the sea of text.
6) The LAN save file will remain the same unless you save over it. As you can just quit with your character after picking the items up, the file will not update. This technically allows you to infinitely dupe whatever items you wish. My advice is to treat the LAN save files like a database. Once you've found the item, you can just add it to whatever character you want. The result is the same, except you don't have to go through the cumbersome transfer process again.

Thanks a million for this! The lack of shared storage is such an odd limitation in this game

Not so odd, shared storage was a design that was only introduced by newer games than Sacred 1. The original Diablo 2 didn't have it either, so people also had to go through Multiplayer games with trusted friends for their transfers back than.

Sadly we will probably nevel get an updated version of Sacred with some new features added, like Diablo 2 resurrected.

There is a Sacred Reborn mod. I've seen some lets play vids on YouTube using it. My first impression is that it has made character progression too easy, at least for the battle mage I watched.