When posting trades, instead of having to click the "Select" buttons on the two different sides, why not have 2 columns of items. This would look like the column listing for the TO Buying tab except that the left column would be your current inventory items, and the right column would be what you are trading for.
Then under each there would be a search feature. If I only want to trade for cauldrons, then the right search would have "caul" so I can quickly/easily select the cauldrons. Then the left column I could change the search feature to what I want to sell. I could have "br" on the left search so I could do Bread, Brew, Bronze Bar, Bronze Sword.

Below the search would be the quantity listing. Forget the slider bar. Unless you have <10 items, I doubt anyone (or at least very few) actually use the slider bar. I always type the number in, even if it's 5. So below the search would be quantity selecting for both Selling/Trading.

Then after posting a trade, the numbers stay. If I want to post 4 trades of 10000 units of water for 10 units of iron ore (this would be 4x 4, so 16 max trades), I shouldn't have to retype all those numbers in again as well. Only if I change the item should the counter be reset to 1.

And then for submitting, have the option at that point of being submitted through gems or coins, not before you even start.