BeastOnaPotato

Each item line by line, it should be fairly quick. 
I'm not aware of a way to add all X to the list 

Options > enter advanced options> vendor and selling > do not sell list. Add the skinning knife to that. 
As for skinning, there are a few threads on configuration, if you have enabled skinning in basic options and it's not skinning, increase the latency settings in the advanced options.