This technique takes classified advertising to a new level. When you send a solo mailing to the subscribers of a closely related newsletter, you will receive highly targeted traffic.

A solo mailing or solo advertisement is a special mailing to the subscribers containing only your message. Many newsletters will allow you to send a solo mailing to their subscribers and they will usually charge a premium compared with paying for regular classified advertising in their newsletter.

It can be worth every cent. One of the weaknesses of regular classified advertising is in just how few of the subscribers actually read far enough to see your ad let alone click through your ad. For a typical email newsletter only 25-30% of subscribers will open the email. And depending upon the ad position perhaps only 5% of those will read so far as your classified advertisement and take action.
When you send a solo mailing you’ll be more likely to have 30% of people opening the email containing your message and therefore a much higher overall response rate.

If you already have good traffic numbers you can probably barter for a solo mailing exchange. If your own subscriber base is much smaller than the one you want to mail to, offer the owner 2 to 3 solo mailings to your list in return for one solo mailing to theirs.

See also Classified Advertising and Advertising Swaps.