I think these are the major ones:
1. Poor blister packaging design: the insert is too tight and too close to the model. When the model wiggles around it gets contact with the blister causing damages
I never thought of this as a major contributor. There have been some isolated cases I recall (some issues with JC 747 NLG, and one of NG's L-1011 lower antenna).
2. Poor use of packing materials: often time I see sellers use the cheapest possible materials to pack models. That is the box, and just the box without anything inside to protect
Oh yes... In theory, the model box should fit tightly inside the shipping box, it is efficient. In reality, we have to assume that the box will get thrown around during shipping, hence extra space for padding material is a must for me. Still, you can always find that fella that will throw the box so hard that no amount of padding will help...
3. Poor handling by the carrier. Always wonder what's behind the scene of their ways of handling packages...
You don't want to know...
So the model might suffer "shockwave" during transit and breaking things apart
Yes, this is what I was talking about when I said sometimes no amount of padding can protect a model.
Slot-in wings and accessories. Unfortunately, seamless molds and extra accessories have become the standard, but that introduces a ton of points of failure. Modern models are very delicate, they are bound to get broken from time to time during shipping.
It is a bit of a philosophical discussion though. Visualize this scenario: A retailer places an order for 20 models from a distributor. Out of those 20 models, 3 come damaged from shipping and can't be sold, or if they are sold, it is likely that most customers will return the model. The retailer then makes a complaint to the distributor about the broken models. I will tell you, some distributors will offer a solution, but many won't.
They are in the business of pushing models out to the market, and many just don't want to hear about the missing antenna on XYZ model, for example.
You think some retailers pack poorly? All distributors I have dealt with usually fill the boxes to the brim with no padding at all (it is just how it works with wholesale quantities, but still). Often boxes get punctured and it is not rare to find the damage transferred to one of the model boxes inside. Again, customer support in these situations is just not part of the culture in this "industry." Gotta have thick skin and learn to eat up the costs associated with those issues from time to time. I'd imagine it is no different in other retail industries.
But let's say the distributor does the right thing, takes the model back from the retailer, and passes the issue over to the factory in China. Who is responsible for the cost associated with the issue? The QC person at the factory? A particular worker? Will they get pay-deducted to cover the cost caused by broken models? Not to mention that the person responsible for the issue might not even be a careless or mal intentioned individual. Everybody makes mistakes once in a while. I will leave you with an analogy here: most pilots always want to make a perfect landing, yet it clearly doesn't always work.
It is a frustrating part of retailing. Even if you want to give good customer service, there is not much you can do when the models are already in bad shape by the time they get to you.