I strongly suspect that the item I saw for sale a few days ago on eBay, labeled "Rosary Beans", was a typo.
But maybe not.
As naturally formed "beads," various seeds, beans, and fruit pits have been used over the centuries for just about anything beads can be used for, including rosaries. (There's an interesting website on bean and seed beads from a botanist's point of view here.)
Obviously, not all types of beans are suitable as beads. In order to make a good rosary bead, a bean or seed needs to be pleasantly shaped, firm, dry, and not prone to spoilage or splitting -- which means ordinary edible beans of the type used for soup are far from ideal, since they are specially selected to have thin skins and split easily.
I don't think anyone knows how old the Hindu rosary is: it almost certainly pre-dates the Christian era, and the Western rosary may have been derived from it (or may have been an independent invention -- no one really knows). But from at least the 11th century, worshippers of Shiva have used rosaries (malas) of Rudraksha seeds. These are the hard, somewhat prickly fruit-pits from Elaeocarpus or "blue marble tree", a tropical tree distantly related to cacao and cotton.
I don't have a lot of firm dates for when or how other seeds came to be used in rosaries, especially since surviving rosaries that appear in collections without documentation are very difficult to date. Some of the most suitable seeds are not native to the area I focus on (Western Europe), and not likely to have been commonly imported until at least the early Renaissance.
Seeds that have, like the rudraksha, some sort of sacred associations are especially attractive for use in rosaries. The "bean" rosary that first attracted my attention to this topic, in fact, is made from seeds referred to as "spina Christi," supposedly from the tree used to make Christ's crown of thorns.
These are flattish, very hard, bean-like seeds, probably from Zizyphus spina-christi, a small thorny tree common in Palestine. I'm still searching for better photos of these seeds to see if that's really what they are: the one really good photo I've found so far of the tree and its seeds shows seeds that aren't as flat or as smooth as the seeds used to make the rosary appear to be.
Another popular rosary material from Palestine is olive pits, especially those associated with the Mount of Olives. I have a very nice modern rosary whose Ave (Hail Mary) beads are olive pits, and whose marker beads are mother-of-pearl. The joining piece is a Jerusalem cross. I've also seen rosaries that are described as being made from olive pits, but whose beads are smooth -- either the olive pits have been polished, if that's possible, or else this is a mistake for beads made from olive wood from Palestine, which are also quite common. Here's a closeup of my "olive pits" rosary.
Last, at least for the moment, is a type of seed rosary that I've seen several of over the years, without really knowing what they were or where they came from. The first one I saw that had any kind of description or distinguishing mark was this one, which includes a medal of Pope John XXIII.
I've since seen other examples, and I became intrigued enough to bid on one at eBay, but owning one hasn't gotten me any closer to knowing what type of seeds these are. I feel particularly embarrassed about this as I have a degree in botany -- though I haven't used it in many years.
I can, however, say confidently that this is something in the "umbellifer" family of plants (its old name, now call the Apiaceae). This is the family parsley and Queen Anne's lace are in, and their seeds tend to have lengthwise stripes, which are actually little tubes full of aromatic oils. Unfortunately many of the seeds look very similar and are hard to identify. And this is also the family that contains poison hemlock, so nibbling on unknown umbellifers is seldom a good idea!
The one tenuous scrap of information I actually have about these seeds is that (or so I'm told) these are special rosaries made from seeds gathered from the Pope's summer "vacation home" garden. That would explain why they're not very common. I will suspend belief, however, until I see more evidence.
*this title was inspired by a cookbook, Bean Banquets from Boston to Bombay. Quite a good cookbook, too.