Just curious, as a guy with decades of sales and marketing experience I'm curious what you think will motivate a buyer to go through the hassle of setting up a transaction to pay above market price for a single PLEX? You're competing against professional sellers who can offer repeat business to repeat buyers who sell their product at a lower price. Whenever I successfully marketed or sold a product/service, I offered something better than the competition. What are you offering to motivate a buyer to do a one-off transaction?
Markets and buyers defy logic. I've seen items on eBay never move at one price, get relisted at a higher price, then start a bidding war. If that kind of stuff happens on eBay your approach may work here.
Your thread seems like an Eve player who has a nice piece of loot and sees a big spread between sell and buy price. With only a single piece, you'll probably have to go for the buy price from the [WTB] threads...