Also, in my experience, I buy stuff all the time, but I rarely sell anything.

I think it's plausible that this is a major component, possibly even the entirety, of the basis of the endowment effect. Selling one's possessions is, in general, an unusual activity which requires a break in routine, possibly stepping outside one's comfort zone, and usually requires more effort than buying something. The differences between WTP and WTA may be due at least to some degree to the different levels of inconvenience between the processes.