What is meant by a structural short in this context:
"First lesson: Valuation shorts are pretty difficult. Look for structural shorts instead."
A "Valuation Short" is a short idea based solely on valuation (fundamentals). This is presumably a "bad" idea and one of the quickest ways of loosing all your money.
A "Structural Short" is said of shorting a company with a mature business model in decline, or shorting a stock that is becoming obsolete or significantly less competitive due to some major changes in the industry.
A good example of a structural short is shorting a "newspaper" stock. The case of the newspaper industry is a good illustration of structural decline: due to the proliferation of online news sources, expensive news priting has become obsolete and has lost its competitive advantage.
So, here is my interpretation of the quote "First lesson: Valuation shorts are pretty difficult. Look for structural shorts instead.":