They don't need an "informal market" when there's a perfectly good formal market. You can sell some of your IPv4 allocation to another LIR party which needs the addresses, the only case that you can't do through the formal mechanism is speculation. You can't buy a /16 and then sit on it hoping to sell it later when the prices are highest before a collapse. Good.

One thing you don't see enough of yet is providers charging for the IPv4 address their customers probably don't need. Those have a real cost, and economics 101 tells us if something has a cost, even a small cost, and you surface that cost to your customers, suddenly they're a lot more interested in helping avoid that cost than they were when you just ate it as part of the service. Plastic carrier bags weren't free back when grocery stories didn't charge for them -- they just absorbed the financial cost and externalised the environmental cost. Charge for the bags and suddenly customers remember they already have a bag, they don't need a bag. Same with IPv4.