Would be nice to be able to have quick filters for "buy only" and "sell only" as well as a filter for currency. Most of the time you are either interested in buying or selling, not so much both at the same time unless you are specifically looking to make a profit by transportation.
A currency filter would make it more viable to operate multiple shops in a non-mainstream currency, as it would allow others to see all the items listed for sale to better judge the value of that currency.
Also show the maximum number there is room for and the maximum number the shop owner can afford if these numbers are different from the current wanted number. In either case it may still be possible to sell more, but that would require buying something to free up room and/or revert to a barter type of trade.