Jo-Ann, there isn't any rule. It is a negotiation: my sofa and love seat for your table and chairs. It depends on what you have, and what you want, and what it is worth to each party. Generally, IMO, 3 cuttings is a nice number, about 8" long and of good quality. We all know that not every cutting is successful, so one only is probably a little "light", unless that is all there is. Then there is rarity, how much you want to offer the get that "spacial" variety, and vice versa. Throw is a bonus if you have extras, or something else that you have a large quantity of. Again, it is a negotiation where both parties are happy. I think the bigger issues is to send quality cuttings. This is often the issue with buying on Ebay: the cuttings were too small, dried out, too green, odd pieces with few useable buds, etc.
Edit: The point of trading, or selling for that matter, if for the recipient to have a successful experience. We can't always control their techniques and success rate at rooting, but we ought to send them enough material that the can be successful in rooting, and thus acquiring a particular variety.
When selling plants from the nursery, I un-sell a lot of plants. That is, I try not to sell a plant to someone for whom it is not suitable, because of location, season, climate, etc. I want them to have a good experience with their new plant, especially if they are new to figs or bananas. It is a bit of a fiduciary responsibility.