So I generally agree with you on (a), but I do like to just try stuff and measure how well it works, and sometimes I get surprised. I did try (b) also, and I actually had a pretty hard time sorting out where to post where my promotional posts wouldn't instantly be removed, how to post in a way that made it clear I'm here to sell my stuff without being overbearing about it, etc. I actually did figure it out eventually and had some level of success with it in terms of people engaging with my stuff, but it didn't lead to any sales (for the short time I did it). I mean, it makes sense. Most people don't read a post and come away from it with the idea "I gotta run out and buy that thing!"

Some subreddits actually specifically say that if you want to advertise your stuff, that's what ads are for. I've got no problem with either paying for advertising or just being honest about what I'm there for; mostly I just care that it works, which is far as I can tell reddit's ads didn't for me.