For example, one can trade away combat skills, and suddenly their avatar forgets how to hit a target effectively. A miner sells mining skills, and suddenly cant mine nearly as well...
What difference do you see between that and selling off strength or agility?
From what I know, memories are just synaptic patterns between cells in the human brain. In the world of EU, skills can be seen as experiences and memories of the avatar. Their experiences of target practice, manufacturing, and mining improves their ability to do those activities, the same way that we humans can with practice.
The difference in the world of EU is that the technology exists to read, store, and write this information. This ability is demonstrated during the reviving process, as bodies can be reconstructed and memories reimplanted from dead bodies. However, as there are no armies of clones made from the most skilled hunters, we can also assume that the reading, storing, and copying processes are imperfect. In reading the memories of a person, those memories are erased in the process. The memories can be stored inside skill implants, but since they can only be read once, we can assume that the storage device uses a biological matter similar to brain tissue to contain the information (rather than digital storage devices), and hence when the memories are read from the implant, the data is destroyed in the process. These limitations prevent memory or experience duplication.
At least, this is the story that can be made up with the rules MA have in place.
Now how is this different from making a person stronger, more intelligent, or have faster reflexes? Physical characteristic are shaped by utility - person who repeatedly use their strength, reflexes, and mind build up thicker muscle fibers, more responsive nerve connections, and more complex brain patterns. All physical changes rather than malleable patterns, and they can't be transferred as described in my above post.
That being said, I suppose if you cut your head off and grafted it onto the body of a body builder, you'd be stronger... but that would be surgery and you wouldn't get more intelligent unless you replaced your brain... how to do that hmmmm