Got this question from a new member, and have no idea what the answer is. Please enlighten us, O Garden Sages.

Thanks!
Mary (and the new member)

Just T-mail them with the request. If the member is willing to sell or trade then, they can work-out the details between themselves, by T-mail.

Does the forum member show it as available for purchase or did the new member see it in their inventory?
If the plant is something that is listed as available, they simply contact the member through t-mail.
If the member has it listed in the Co-ops, Group Buys and Classifieds Forum, then they either post in that thread or again, contact through t-mail.

Jay beat me to the answer as I was typing.
I wanted to mention that if the plant is simply in their inventory, generally I would think it's not something for sale.

>> How do I purchase a plant from a forum member?

If you find the plant in our Plant Database, look for a box labelled "From our plant lists:"
Then click on "» 1 member has this plant."
In the "Status" column you'll see things like
"Owned"
"For Trade"
"For sale"

But note the red infobox :

"The plant lists feature is where members keep track of what plants they own (or want to own).
If the plants are available for trade or sale, it will say so under the "Status" column.
Please don't ask the member for plants unless they specifically say the plants are available."

I'm not sure how @Dave feels about dropping wistful hints in a forum thread.

Or you can go to that plant in the Plant database, and click on the blue button "Add this plant to your list". It's 1-2 pages down, in the far-left column.

I think you can add it to your "Want" category, or you might have to go to your own "Plant List" management screen and create a category for "Plants I Want" or "Want to buy".

I think that more members trade plants or give them away for postage, than actually sell plants.