All I can do is relate my experience. I am in a similar situation to yours in that I don't expect to make a living from photography but had accumulated a bunch of reasonable shots that I thought were worth something. During most of last year I was not working so I took the time to submit some pics to first one, then eventually 3 micro-payment sites and one not-so-micro site. I'll write about the micro experience. Most of the shots were taken with a 4mpix higher end digicam and the rest were from down-rezzed scanned slides. The reason for the downsizing of the slides is that I don't believe it makes any sense to sell high-rez versions of photos for such low dollars. Micro-sites target a specific category of buyer and I think it's best to aim at that.
At first I had a lot of rejections, until I bought NeatImage and also started to sharpen more aggresively. I found it odd that micro-sites to which I was submitting 4 mpix shots would be so picky but I guess they have to draw a line somewhere and they chose to evaluate pics at 100% screen magnification. This seems like overkill to me, given the likely size of final print, but it's their site and they have to set some standard, I suppose. So far as that goes, it's good practice to prepare photos for them.
I have about 200 pics at two micro sites, and 65 at a third. I have made (my cut) $60 in about half a year. I look at the statistics on the site and there are others who do much better. Since my shots are of things that I happen to come across and like, they don't necessarily match the demands of the clients of those sites. Hence, my low sales, relative to others. Spend time looking at what's offered for sale by contributors with high sales volumes. It's not pretty landscapes, in the main, not at the micro sites, I don't believe.
OTOH, my pics were just sitting on a hard drive so getting anything for them is a bonus. Also, the incremental cost of taking and submitting more pics to them is nearly zero, so I keep doing so. The inventory slowly increases, and it's a little like compound interest. You don't think about it, and then one day you're suddently rich. Sort of.
The amount of money I've made is laughable, of course. But then, the number of pictires I have for sale is small as well. But it's good practice. And it's not as if my other choice was selling framed prints for $400 apiece. It costs time to prep and submit the photos, not to mention the time it takes to shoot them, but part of that you would probably do anyway, and part of it is educational as well. You never know where it will lead.
RF micro sites generally get a lot of criticism on photo boards but they have their place. You are not likely to make a lot of money with them, although with enough effort you might, but you'll make something.