If trading assistance is otherwise profitable, and worth your while, to you then you might want to adopt a strict rule of having all merchandise on-hand before you list it. There's no reason to throw the baby out with the bathwater.
Whether or not to suffer the travails of being a trading assistant is a moot point for those of us here in British Columbia. The Trading Assistant Program in B.C. has never worked due to software glitches. I signed up many years ago to be a trading assistant and never got a single enquiry. Then, a year or two ago, I tried an eBay search for local sellers and the closest trading assistants found were in Washington and Alberta. I just tried a search again, right now, and the results tell me that trading assistants in Korea, Scotland, and Houston Texas are all within one mile of my home. How much potential B.C. business has eBay lost, year after year, due to this software problem? Is B.C. the only such area that turns up aberrant results?