Most Common Aspect Ratio?
The two most common aspect ratios are 4:3 and 3:2. You will also see a significant number of 1:1 (square) photos and 16:9 ("widescreen") images.
3:2 is the aspect ratio of 35mm film cameras, and that has carried over to most DSLRs, both the APS-C size and "full-frame". Most compact digital cameras, along with the Micro Four Thirds interchangeable-lens mirrorless system, use 4:3.
Most smartphones are also normally 4:3, but popular apps like Instagram produce square 1:1 images, so you can't discount that. And, the HDTV-like 16:9 aspect ratio is a common option in many cameras of all types (usually but not always just by cropping off the top and bottom of each image in-camera).
It would probably also be wise to consider the possibility of arbitrarily-wide panoramic images, as many cameras (both via apps and just as a feature) have the ability to produce those very easily. And, of course, you really shouldn't discount cropping, both to historically-standard ratios like 8:10 and 5:7, and completely arbitrarily.
And keep in mind that you will also see these turned the other way: 3:4 and 2:3 for portraits; 9:16 looks odd and is less likely, but the theme here really is that anything goes.
This is all off of the top of my head, but if you'd like the referenced and researched background on these aspect ratios and other common ones found in photography, see my answer to What historic reasons are there for common aspect ratios?
Most of my users will be using a digital camera (they will be taking photos of products they are selling). So, should I be focusing on 3:2 or 4:3?
I think you reasonably need to cover both, but if you're sure that DSLR users will be in the minority you can probably get away with a focus on 4:3.
In that case, go with 3:2. That's the ratio used by Canon, Nikon, Pentax, Sony, and Sigma DSLRs. Only those from Panasonic and Olympus use 4:3 (and the latter companies are now focused on their non-DSLR system cameras).
AS long as you know that they won't take pictures with their iPhone you should be fine with 3:2 :)
Some will use iPhones, etc. I need to deal with the most common aspect ratio for the design of the site.
If it's any help, I just queried all photos I've taken over the past many years and here's the first 25 results:
aspect_ratio count 1.33333 34312 0.75 9784 1 6929 1.5 5062 1.77778 3175 0.5625 1528 1.33628 763 1.50588 551 0.666667 521 1.49876 515 0.741722 454 1.49927 367 0.746875 363 1.25 355 0.665278 331 0.1 288 1.33891 281 1.90099 279 0.726562 265 1.50313 234 1.5047 209 1.50623 190 1.50905 182 1.50104 180
Results will be from many different cameras and taken by a few different humans, but me and my friends/fam of course do not represent the entire human population.
This answer would be better if you condensed the rows that are clearly the same aspect ratio (or at least nominally the same). For instance, 1.33333 and 0.75 are the same aspect ratio, 4:3 (the only difference is orientation: the first is landscape, the latter is portrait). Same with 1.5 & 0.666667 (3:2). Also, all of the 1.50-ish ones are all _nominally_ 3:2.