Another consideration to take into account is price point – what is your audience willing to pay? Believe me; you're unlikely to make sales promoting products and services that are outside of your audience’s money comfort zone. Honestly, it took me a long time to narrow down who my target audience was, but if you’re focused and take the time to do the research, it shouldn’t take you that long to figure it out.
But I think the biggest deciding factor in this, goes back to the site as a whole and all of the other posts. Are the genuine? Is the blogger constantly trying to push products? I’d like to think I’ve been doing this long enough that my audience knows I’m not out to make a quick buck – and I think even relatively new bloggers can prove this based on their other content.