Today I stumbled across SBL listing SBL120550, which says the following:
"Several IPs in this /28 are sending spam to spamtraps advertising the services of ADT Home Security. The IPs belong to InfoCanada, a division of InfoUSA, via their Yesmail ESP.
InfoUSA also sells purchased and e-pended lists. We do not know whether the purchased list that the customer is using is using came from InfoUSA, but we consider the sales of purchased and e-pended lists to be spam support by definition. Use of such lists is a reliable path to an SBL listing." (Emphasis added.)
There you have it, straight from Spamhaus themselves, explaining exactly what they think of purchased and e-pended (email append) lists.
Laura Atkins of Word to the Wise has compiled some very helpful ISP Summary Information, showing that, for starters, the SBL is used as a spam filter at AT&T, Comcast, Cox, RoadRunner, and Yahoo. Meaning, use of email append can lead to a blacklisting by Spamhaus, which leads to blocking at those ISPs. And they're not the only ones who use Spamhaus; I think Hotmail and Gmail do, too. Not to mention, many other smaller ISPs and corporate sites.