You didn't buy c**p, but you did pay too much. It is a nice Art Deco (1920s-30s) silver and marcasite brooch. Not top quality, but not rubbish either. Are there any marks? They can be tiny.
Yes, that is an assay mark. It looks like it could be the French Minerva head, but let's wait for the others, my eyes aren't too good.
A French mark fits with the trombone clasp. Paying that much is fine, although it feels like too much, if you really like it and want to wear it. It's about 5x too much if you want to resell it.