+1, I think that could be easily doable, but would most likely have to be some sort of plugin that is made to do a similar thing as the plugin for turning chests into shops, but instead turns the block into a 'merchant_sell' type thing.
it would probably require some sort of way to know when to fire the command, such as
A; trigger after no more influx
B; Block is full
C; Redstone signal
or just simply D; timer set to sell every like 30 seconds.
Hoppers are slow fortunately, but unless there's a limit on where/how many of these 'sell blocks' you can have, or how often it sends the /sell signal, might have a bunch of these just sending out pointless/wasteful commands repeatedly and bogging the server down a tad. Would need to limit it per # of people on a claim or something.
+1 as well of the block taking some of the sale, you are losing $ for convenience. Money doesn't have a huge use right now so might as well partially pull back on the inflation with a block cut lol.