I've got some F4 Blackstix I'd let go for $50, too many composite quad sets to name that I'd sell for $30. All new or lightly used.
I thought they were, but Futures says they are small. I've ridden them in all sorts of waves no issues. I also have Shapers S5 fiber flex, new in package, which ARE medium. $40 for them. http://shapers.surf/fibre-flex-s5-1/
I've got two set of Roberts Next Generation Future tri fins sets. One is medium (red) and one is large (blue). Each used once, perfect condition. $85/set shipped USPS priority anywhere in the states or $75 if you are local (Brooklyn or Long Beach, NY). I don't think they sell the mediums as a tri fin set, Robert broke up a quad set for me when I ordered a board.
Pumpmaster, you still got those S-5's? I broke the left fin off my board last night. Looks like the same exact fins from the picture.