Many regular farmers find this much easier than waiting for the items to sell in the AH or having to relist them. This way you get a good supply and he gets paid immediately. I met a JC who did this with saronite. He would buy everything the guy had in order to prospect it. The guy got rid of everything and got paid immediately and the JC always got his ore for a fair price, still making an excellent profit.
Now, don't mess with folks. If you say for 1 month you will buy every stack of Lichbloom they have, commit to that and don't play games. If they are mailing the stacks COD to you, don't let them sit there without paying the gold immediately. Its unfair and you will lose your good farmer and good rate for your herbs.
Start this process by doing shoutouts looking to purchase 10stacks of lichbloom and once you find someone who has that many, make a prosposal.
I'm sure this will help keep you in mats and keep your expenses in check.