It's hard to distinguish between leaking and selling, but I think leaking is much more common. Dropbox famously leaked a lot of emails, including mine - I was never a paying customer and that put me off becoming one or using them. Two local PC parts companies leaked or sold my email. I confronted one about it and they claimed they hadn't had a data breach, so either they sold it, or they were too incompetent to know they'd been hacked, or they lied - I suspect incompetence but whatever happened they lost my business. A couple more incidents long ago too.

Real estate agents can be pretty aggressive with emailing, but IME respect unsubscribes and don't seem to share/leak emails. I kind of wish I'd used an address per agent instead of per company to see what was happening better.