Oese and CallSmall are to thank for turning me on to N scale railroad model miniature dollhouses. The yellow house is Bachmann N Scale Contemporary House - I paid $13.99 on ebay, but I saw it online at hobbylinc.com for $9.49. The green house (pictured front and back) is a Herpa two story flat - I paid around $7 on ebay, there is a set of three similar white ones on ebay now, I haven't seen these online. Lastly, Kato Structures 7410 N. Clarmont House #23-405, I paid $18.50 on ebay, but it is online at katousa.com for $22.50.
Here are some great pictures from their blogs that got me interested. The collection of houses on the shelf are from Call of the small. The two interior shots are from Raum für Raum.
Here are some others on ebay that I almost bid on. Green set of 2 Vollmer N Scale Model Apartment Buildings, sold for $22 (love these). The High Rise building and Kato N Prof Building/Mama's Seafood KAT23406 are still up on ebay.
(images from ebay, Raum für Raum and Call of the small)