It'd be a great idea to allow someone to answer their own question on the forums.
For example, I get frustrated sometimes and post a question to see what the community has to say about the thing I have run into. I then usually go back to working on the issue and sometimes stumble across the resolution before someone else provides an answer that solves it.
Currently, I am not aware of a series of steps that I can take after posting my answer to allow me then to mark my own answer as the correct one.
I think this would be a handy feature to have and would increase the hits on answered questions that are similar.
I totally see the validity of your suggestion. At the same time, there is so much of misinformation in some of the sub-forums that disturbing. Even worse is that the original poster is believing the incorrect response as correct. Given this, allowing one to accept as correct one is risky, especially in a forum that is not moderated.
I do feel your pain though. My request is to document the answer and move on. Some of us who monitor few usually recognize such efforts and tag them as "Correct Answer". Here is a recent example: