I know where you are coming from OP, I've had similar experiences (even pictures can be deceiving), but I'm not sure a weight system will work. I mean it might work for stock standard tricho cuts, but it doesn't take into account: the rareness of different plants, the fluctuating weights of cactus, and the different weights between species. Plus as you said there could be issues regarding legality (cuttings are borderline as they are). If a seller isn't willing to provide you with more pictures or more info just don't deal with them. If they are dishonest in their dealings and unwilling to fix the situation then you can leave feedback in their trade rep thread.