Any of these is ok, except I would simply say "I used to post as X"; the "before" here is redundant. However, to do what you want, I would prefer not to use any of these and use formerly instead:
For completeness's sake, "I was posting as X before" means that over a period of time in the past, you had the ongoing habit of posting as X. "I used to post as X" means pretty much the same thing, and I would prefer this usage. "I have posted as X before" means that at least once in the past you posted as X.