Hello , can you say the plugins : Shop where are things on slabs Plugin that who has permissions can walk to place [ like VIP zone ]
A shop plugin like this? http://dev.bukkit.org/server-mods/chestshop/ And for places for VIP-Players use: http://dev.bukkit.org/server-mods/worldguard/ Protect Areas and set flags, iam not the best in Bukkit but this my ideas!!
Second is fine (Flags), however with first he meant that you place flying block on slab, and hit the slab with left or right click to buy it. I had that plugin long time ago, it was called showcase shop, or something like that.