Thanks for bringing this up V. Mine was locked also and yes you kinda of feel like you did something intentionally wrong. I have never intentionally bump my for sale thread. I sew enough without doing so and really don't want to sew everyday!
Maybe a warning first through a pm ,that way you could see what you are doing wrong first and be given a chance to correct it.
I have seen other for sale threads with comments after comments being address and not locked so it is alittle confusing to me.