The IP trading market is steadily growing. However, there are many different reasons to buy IPv4 address space. Deciding to buy is an important consideration, especially for businesses that still make heavy use of IPv4 (nearly all of them).
The Current State of IPv4
Understand there are no new IPv4 address blocks for ARIN to give away. If you're a provider, or a data center, that currently relies on IPv4 address space, you have to make do with what you have, or buy new blocks.
Why not just go with IPv6? And that's a fair question. Even while you make the transition, you will still need your IPv4 addressing in place. IPv6 implementation can take some time depending on the size of your business. And its adoption rate isn't anywhere near universal at the moment.
What are the Reasons to Buy IPv4 Blocks?
Just to stay in the game, you still need your IPv4 address blocks for the time being. Not only that, but you may need a few more blocks as well. Consider the following situations.
You're near saturation – If you already have enough addresses to handle your business needs, then buying a small block may shore up any eventualities. For example, app developers that have already peaked in user numbers don't particularly need any more blocks, but can use a few more to handle sudden spikes in users.
You're still growing – If your business is still growing and you anticipate the need for more IPv4 addresses, then you should get them now. While you work on implementing IPv6, your requirements for IPv4 aren't going to go away, especially if your growth is steady. You will need to buy up blocks to accommodate those needs in the interim.
Even purchasing a small /24 block can go a long way for a business, especially if they split up those 256 IP addresses using NAT. In the case of smaller businesses, it's possible that the extra block can see the business through the next ten years or so.
That's plenty of time to switch to IPv6 without worrying about your IPv4 address space running out. Just keep in mind, there's a good chance that you will definitely have to switch as IPv6 comes into its own.
Use an IPv4 Broker
Many businesses aren't sure how to purchase IPv4 blocks outside of going through the official channels such as ARIN. IPv4 brokers facilitate the trade of IPs between those that need them, and those that want to sell them. Even if you cannot buy, it's possible for a broker to help you lease addresses from those that own them.