Bid low on the "trash", get it for cheap, and sell it back to get gems.
So trash's use is to be re-sold because its trash? It would be better if it was just removed from the AD, or 2 bids allowed to go at once, or the shelf increased in size.
Cause CursedVeggie is right, you get hundreds of gems, its not like you need more, and you get hundreds of thousands of coins waiting for something good to appear anywhere in it. You don't need to waste time buying trash to re-sell