.io is being hyped a lot lately. any domains that are making a killing in .com like NNN.com n NNNN.com will almost certainly not make a killing as .io, and i fact, while NNN and NNNN in .net and .org have also risen in prices they're no .com's, so first I'd study the resale market for .io domains, if there really is one. Beware of any new TLDs esp if you just started domaining