You also point out another market niche -- customizing modules.  Many of the modules available fit the "close but no cigar" category.  And while some may buy the source and do the programming themselves, most places of any size are perfectly happy to buy the module customized to their needs.

Support is another big area to differentiate modules.  I've bought plenty of modules over the years, and it seems there are always a certain percentage of developers who lose interest and abandon their modules.  There are others who are slow to update, meaning an upgrade I need to do has to wait until they have finished their upgrade.  I don't mind paying for support, as long as it's not outrageous (15% to 25% of the sale price is a standard range for support per year).

Jeff