You are kinda limiting your end user base by registering it with a trademark. It kind of only makes sense if the buyer actually wants the trademark that you guessed at registering. Way to many guessed variables here, first the domain name, and second the trademark. A long-shot, at best. I don't think it makes your domain more valuable or easier to sell. So why spend the money?
What a trademark would do is help you out should somebody decide they want your domain and slap you with a UDRP. Of course, YMMV.