I'd be very cautious about engaging with any company whose business model is to get a short-term lease of your IP-space. Many companies use IP reputation data, and so you are essentially lending that reputation to a 3rd party, who may use it in ways you don't anticipate until the reputation is sufficiently damaged, and then return it to you and move on to another ISP.
Some organizations' response to unwanted traffic is simply to block large IP ranges or entire ASes, and not everyone is good about following-up and expiring such blocks in the future. I realize your customers haven't ended-up on any spam/abuse blocklists, but that doesn't mean they won't be, or that their IP reputation hasn't already been affected in less obvious ways. You should ask yourself if you are being sufficiently compensated for these risks as reputable IPv4 space is at a premium, so replacing the IPv4 space you lent out could get quite costly.
--
Mike Fuller :: Security Reliability Engineer :: Google :: AS15169