The reason there's isn't a 'search owner by item' function is that people don't want to be harassed about selling their stuff if they're not clearly trying to sell it.
Sure, maybe 1 time in 20 you'll pm someone with an offer and they'll actually take you up on it but with the market board and automated feature the thinking is that if someone hasn't explicitly indicated that they want to sell their stuff then they don't want to sell it or aren't interested.