ordering custom modular directions on eBay
In this case, the seller is planning to sell you a .pdf, so you don’t even have to use the LEGO Digital Designer software. As long as you have something that reads a .pdf file, you should be fine.
http://www.ebay.com/itm/LEGO-Custom-Modular-Ladies-Apartment-INSTRUCTIONS-ONLY-/142255734530
That builder has some interesting designs. Buying the instructions is one thing, but having the parts to build it is entirely another. Its easy to make something in the LEGO software from parts that are expensive, uncommon or don’t even exist in specific colors.
If you are interested in buying the instructions for building the ladies apartment, ask the seller to send you a list of parts that you would need before you buy the instructions. If he does not know how to do that, ask for the .xml file of the parts list or screenshots of the parts list on the last page of the .html file. That will give you an idea of what parts you would need. Of course, if you want to build an elaborate interior, its going to take a lot more parts.
Of course you could always try to build a similar one on your own based on the photos and parts that you have. You can ask other builders if you cannot determine how the designer did something.