Great idea!
Maybe a plus-value feature could be added to Plant Database search results. A column that would have a "V" or some icon showing that THAT entry has a vendor listing. Or "V3" to indicate three vendors.
Someone searching inside NGA for a vendor might like to see right from the search results which entries include a vendor.
And I would think that vendors would appreciate it / pay for it.
(I thought of that while trying to find any entry WITH a vendor listing, so I could see what they look like.)
Taking it to an extreme, the column might even have "H5 W2 V3" to indicate that five people HAVE it, two people WANT it, and three vendors are listed.
(Plus maybe "C2" to indicate two Comments. If you have list of 30 matches, you might wnat to read the ones with comments first.)
Taking it well PAST extreme, and cross-pollinating it with the Seed Swap tool in a "live" way, how about adding "S1" or "S2" to the new column if it is currently being offered in one or two seed swaps? The database entry would say which seed swap, and when it starts (or if it is running, when the last date to join is).
There would have to be a caution displayed, saying that trade-lists change and the item might be withdrawn before the swap starts, or someone else might get it first. I'd suggest some button that a database browser could click to indicate interest, as feedback and encouragement to people adding packets to their tradelist and trying to guess "what people want".
Maybe these are needless complications. Many of my suggestions are!
Have you considered how to handle plants that a vendor typically runs out of each year? That seems to be a pattern for people selling rare plants. The "good ones" are sold out early. Maybe list only those currently "in stock" (terribly hard to keep current), or list any that the vendor usually sells some of each year? Give vendors the OPTION of removing out-of-stock items "live", if any vendors would remove something that might have generated a "hit" for their site.