Hmmm, species is difficult - the smallest leaved figs I know are F. benjemiana, F. rubiginosa and F. microcarpa. It could just about be any of them from that photo. IMHO, I wouldn't pay $40 for that plant as a Bonsai as it is not that well trained - if it was my plant all of those branches would be overdue for a drastic shortening. I think you could grow a seedling Ficus of any of the species I mentioned above to that stage within five years. The main problem you will have is the need to keep it indoors when the weather is cold - Ficus are fairly tropical and don't grow very much indoors. If you want to bonsai them you need a lot of growth so you can keep trimming back to thicken the caudex.
"caudex" in the sense we use on this forum, ie "a swollen organ at the base of a plant consisting of stem or root tissue, more or less exposed to the air and used for water storage during periods of dormancy" is not actually what you get in most Ficus species. The basal swelling you can see is mostly woody, with a relatively thin layer of live tissue over the top, It's main purpose is to stop the tree falling over, since the actual roots are relatively shallow for trees which attain such size. There ARE some more truly caudiciform Ficus species - here is a good example from Plant Files: http://davesgarden.com/guides/pf/showimage/49258/
To be truly frank about the pictured specimen, I just don't like the look of it, and I certainly wouldn't spend that kind of money on it. No doubt, someone has put some time into it, but obviously with little guidance and knowledge. It makes me wonder WHY they are trying to sell it at all. Firstly, it really has been raised too far up out of the soil medium in an attempt to make it look "interesting" and "bonsai-ed". Secondly, the sphagnum moss surrounding the tree is all wrong - it belongs only with plants which live in a bog type environment, so it is either dead, in which case it shouldn't be there at all; or, if it is alive, the plant must be drowning in water, with the usual consequences for the state of its roots. Thirdly, as I said before, all of those long wispy branches should be cut right back to within an inch or so of the trunk of the tree.
In general, most Ficus probably don't mix well in a purely caudiciform collection - They will tolerate a dry season, especially as adult plants, but most don't ever go completely dormant and loose all their leaves so they require a year round regimen of watering and sunlight to flourish - it wouldn't actually surprise me to find that many of the Craigslist trees apparent problems stem from just that factor - too much time indoors and not enough light
I got you all. I am a real caudiciform novice. I only know about plants I see in a few nurseries, personal collections, or on line. But I troll Craigslist for plants constantly to see if some one is selling or giving away something good.
Which is exactly why I offered this up. I will pass on it, and I appreciate your opinions.