In my opinion, it would be unethical to ask for cuttings for free or for shipping costs with the intent on selling them for a profit. That is just preying on others' goodwill. Also it's hard to see how that would be a very profitable business model.
There are other situations where there's probably a more diverse set of opinions: For example:
1. If you negotiate a price (in excess of shipping costs) for cuttings, and then sell them for more on Ebay, I don't have a huge problem with that. That's just how business works. If I found out, I would just probably negotiate a higher price the next time. Or sell them on Ebay myself. But I've sold cuttings before, and it's a lot of work and hassle, so if someone wants to be in that business, they should be compensated for their effort.
2. Anyone who has grown figs knows that it's impossible to predict how many of your cuttings will survive. So if you start 4 cuttings, they all survive, I see no problem with selling the extras. Or even all 4 of them. Like another poster said, you are adding value so you should be compensated for your efforts.
3. The keeping 2 and selling 2 example is a little tricky. Did they specifically ask for 4, with the intent of selling 2? Or did the not even think about the number they would receive, and then when they got the cuttings they figured they had a good shot at getting the tree with 2 cuttings and decided to sell the extra? If the latter, I wouldn't have a problem with it. If the former, then that's a little shady. But if the cuttings are so valuable or rare or sought after that you can make a substantial amount of money by selling 2 on ebay, then why are you giving 4 away to 1 person in the first place? I wouldn't do so. That's not generous, that's just silly. Why not give 2 each to 2 people? That would be better.
I think that it is fair that when giving a gift that there are certain unstated expectations of what happens in the immediate future. For example, if someone gave someone a pet, I would think it reasonable to expect the receiver to take good care of said pet and not abuse or (heaven forbid) eat it. Certainly a fig cutting is a much different situation, but you do want that cutting to be used for a certain purpose, namely to start a fig tree.
If you want to state specific conditions when giving cuttings, you are free to do so, but it may be hard to enforce or monitor whether said conditions are met on an ongoing basis, and the only recourse you have if conditions are not met is to not give to the same person again.
I do expect that if I give cuttings to someone for free or shipping, and if my tree dies, that if I ask that person, and the cuttings I provided to them resulted in a tree, and said tree can yield a cutting, that they give me cutting for free or shipping costs if I ask nicely, provided it's legal, etc. Why wouldn't they? However, if someone is enough of a jerk to refuse such a request, then they've probably got a lot bigger problems than I have, and I'm not going to lose any sleep over it.