Perhaps folks can PM you about these boards.  Some, like TER, are free.  Others, may charge you to put up ads.  But some of the ones that charge have reviews (written by others) which is like free advertising.

As far as ad sites (you mentioned backpage), there is eros and cityvibe.  Both are national but are city based.  For instance, the new york area eros is www.eros-ny.com.

Hope that helps.

It isn't super active but a lot of high quality hobbyists do check it out, and it is free and very interesting too.