The primary specialization you should be targeting is this:
- low cost domains,
- that you sell at the highest margin,
- in the shortest span of time
These three ingredients will make you not only successful, but true Domain Sales Royalty if you can perform to the standard with regularity.
Other than this specialization, well… The highest profit margins come from domains under every TLD, of every length, and in every language. Perhaps you could argue that a domainer who only did ".coms" would be more statistically likely to do better than his or her peers… But it would still depend entirely on finding customers for his or her inventory, end of story.
The closest thing you can see these days to a "specialized market" are the newer TLD registries like .co… And while there may be a slight SEO bump in some cases for such domain names, and it's certainly more "wide open" in terms of possibilities than the .com space, it seems a LOT of money is being poured into marketing, with results being okay for the registrar, but nothing to write home about in terms of ROI % compared to a successful domain seller.