Just one week after we covered how Facebook shares were trading at a $17 billion valuation on SecondMarket, competing private market platform SharesPost claims that Facebook’s valuation is $11.5 billion. Does something sound fishy here? There are a number of reasons the valuation could be so different, but most of it boils down to the lack of information in private markets.

I can guarantee you though, if I purchased shares of a company at one valuation, and found out that I could purchase it at a 35 percent discount through another trader, I’d be pretty angry. Before flipping out about the overpriced shares you just purchased on SecondMarket, let’s take a look at how SharesPost calculates their valuations:

SharesPost calculates the average value for each company from its most recent transaction price on the platform; the midpoint level between the latest bid and offer prices; valuation estimates from research reports; and any venture financing in the past six months.

So should we assume that Facebook shares recently traded on SharesPost at this rate? Perhaps. We also could guess that there is a large discrepancy between the bid and ask price currently on the SharesPost market. Sellers may want to sell at $15 billion but buyers want to sell at $10 billion, for example. Whatever the case, the fact that the numbers are so dramatically skewed emphasizes the risk of trading in private markets.

Also, in many of these markets (SecondMarket included), the deal is not completed until a broker finalizes the transaction. When we got an offer to purchase shares of Facebook at a $17 billion valuation from SecondMarket the other week, there’s a chance the broker was fishing for a large investor who would be willing to pay a large premium on the shares. That way, they could pay the seller a lower valuation and pocket the difference.

Ultimately the market for private companies has just begun to emerge. Overtime, the market which provides the greatest transparency will be the one which wins. When the market’s priorities are not aligned with both parties, it’s simply bad for business in the long-term. The best conclusion to take given the latest news is this: unless you personally know the other party in a transaction, it’s extremely difficult to determine what a private company’s stock is actually worth.

While companies like SecondMarket and SharesPost have gained increasing credibility, the system has still not been perfected. The main problem is that the public announcement of valuations could be nothing more than a tool the market owner is using to manipulate current transaction. Would you become a buyer of Facebook shares on the private market? Do you think the large price discrepancy even matters in the long-run?