It happens sometimes with low rarity plants. I had a Fourpetal Ball Cactus that I kept until the 250s, and it just would not sell. I tried cutting the price in half and selling it both with large groups and by itself. I eventually cut off the flowers to see if it had a bad spot, but it didn't have any that I could see. Sometimes a plant just won't sell, so your best bet if it is a low rarity, cheap plant is to just trash it. Higher rarity plants always seem to sell well.