online listing services like ziprealty.com / realtor.com

Any investors out there who use these sites? Any opinions on which one(s) is(are) the best? If they’re not useful for investors, does anyone have any ideas on what could make them more attractive/useful to investors (as opposed to “regular” sellers and buyers)?

The best way to go is to create your own website, using newspaper ads and signs to drive people to the site and the houses you have for sale.

As to ways to make them more useful to investors, they would need to remove the fluff and lower the costs.