Warm market is used in both the sales and direct marketing industries. In direct marketing it is similar to using the phrase "permission marketing", which is also the title of a best selling book by Seth Godin. In sales and direct marketing your warm market is said to be people you know, which could be friends, relatives and existing customers, even referrals are said to be warm (see referral marketing.)
"Cold market" refers to a sales person's future or prospective customers.
References
Warm market Wikipedia(Text) CC BY-SA