Sometimes the market just does not have what you need, and despite your best intentions you end up cluttering chat with requests and offers. I suggest that it be possible to write a book, then place this book on the market to request an item be sold to you. (or something along that general idea)
This would be a major change I believe in the code. While I understand and think this idea is nice, I don't see it happening any time soon.
also it wouldn't work for all items, some things in NEI have different Item IDs from the item you actually get when crafting it. So it wouldn't work for everything, and confuse some players. Generally i don't see a problem from asking in chat, even with >20 players those requests aren't annoying at all (well some users ask every 2mins, but those get told to stop it, and they usually do so then)
Instead of changing the code why not make a section in the market for requests and people put them on there written in books?(no idea if that would be possible or if it would require a major change in the code)
Maybe place some kind of sign/message board in the spawn point somewhere? Or designate a space where players could leave signs with their "grocery" lists and the name. Then everyone could work out their deals via private chat from that point on.
You're talking about buy orders. It's sounding like an entirely second plugin for the most part. I mean, you'd have to pay up front to request an amount, and then it'd have to be fulfilled. Is our plugin a mym original? We might just as well make a forum post for what you want, and what you're willing to pay for it.