You'll want to scale up the number of items along with the number of squares in order to maintain effectiveness. Also, I highly recommend switching from seeds to coins. Coins are almost completely safe.
I'm working on trading enough single coins back to myself to make the room work efficiently now. Only the first year, and the merchants made off with quite a few coins... But I have more than enough metal present to make this work. Just need to get a repeater going, and I'll be set. Don't have a spare dwarf to pull a lever for eternity, so I guess I might as well learn the mine cart way.
You should be able to break down a bunch of stacks of coins with just a couple dozen crafts as bargaining power. The technique I use is to bring several stacks, say, 4. Then I use a macro to offer 100 of each stack. Use cntrl-u to do that to all of the stacks. Trade that for some stuff that looks appealing. Now trade 100 of each of my stacks for 100 of each of the stacks he has, and throw in one craft on top to sweeten the deal. Repeat 3 more times, which is easy with the macro.
Now, exit the trade screen, queue up all the coins in the trading depot for trading, and reenter the trading scheme. Repeat as above, but with 20 coins at a time.
Exit the trade screen, requeue all coins for trading, reenter, and repeat with 5 coins at a time.
Do it all again with coins one at a time.
All told, this should take 5 to 10 minutes. It's kind of a pain, but you only have to do it once. And it scales well as you do several stacks at once, because you're using a macro and cntrl-u - you can split four stacks just about as easily as you can 10.