By George Chen
The opinions expressed are the author’s own.

I remember a veteran trader once told me of the three scenarios under which one should sell stocks.

First, sell when you start to sense the government is beginning to tighten market liquidity, indicated for example by a sudden influx of IPOs or a tougher monetary policy. Second, sell when you see almost everyone, from monks to neighborhood grandmothers, is buying. Third, when you see big banks such as Goldman Sachs downgrade their economic forecasts, which basically means they know they misunderstand something and have to fix the misunderstanding, sell.

So, this week Goldman Sachs trimmed its economic growth forecasts for China to 9.4 percent this year, from 10 percent previously, citing a recent run of surprisingly weak data, high oil prices and supply constraints. Goldman’s report created a buzz in the market, pushing some investors to sell further amid already weak sentiment. More banks are expected to follow Goldman’s move to trim their China forecasts in coming days and weeks.

Will Beijing be happy to see economic growth finally slow a little amid concerns of possible overheating in some sectors, for example, real estate? The answer is both yes and no. Yes, Beijing expects to see some cooling of the economy, but if it extends too far it could lead to massive money outflows, which would be an even bigger headache for the government than high property prices, in my view.

If you don’t trust Goldman Sachs’ forecast, you should trust gold prices — some traders shared a trick with me to forecast the timing of a market rebound. When you see gold (and other commodities) prices start to cool, then it may be an opportunity to buy stocks.