Many people assume that if the price of something is $10 and they have 1,000 of that thing, they should expect to be able to sell them for something around $10,000. Such an assumption may hold much of the time, but it doesn't always. Worse, the cases where it fails to hold are often those where it would be relied upon most heavily. Such an assumption should thus be considered dangerous.
In a liquid market, the quantity of a something that people would be willing to buy at something close to the market price will be large relative to the quantity that people would seek to sell in the short term. If at some moment in time one person in the market was willing to immediately buy 500 shares at $9.98 and another was willing to immediately buy 750 at $9.97, someone seeking to sell 1,000 shares could immediately receive $997.50 for them (selling 500 to the first person and 500 to the second, who would then be ready to buy 250 more from the first person who was willing to sell for $9.97). Such behavior would be in line with what many people's assumptions.
In an illiquid market, however, the quantity of something that people would be willing to buy near market price could be surprisingly low. This is more often a problem in the marketplace of things like collectibles than of stocks, but the same thing can happen in the stock market. If there's one potential buyer for a stock who thinks it's overpriced but has potential and would be worth $9.50, but that person only has $950 to spend, and nobody else thinks the stock would be worth more than $0.02/share, then until people sold a total of 100 shares the price would be $9.50, but after that the price would drop instantly to $0.02. There would be no "cushioning" of the fall. If the person with 1,000 shares was first in line, he'd get to sell 100 shares for $950 to the aforementioned seller, but would be unable to get more than $18 for the remaining 900.
A major danger with markets is that markets which are perceived as liquid attract people to the buying side, while those which are seen as illiquid repel people. The danger in the latter is obvious (having people flee a seemingly-illiquid market will reduce its liquidity further) but the former is just as bad. Having people flock to a market because of its perceived liquidity will increase its liquidity, but can also create a "false price floor", causing demand to appear much stronger than it actually is. Unless real demand increases to match the false price floor, the people who buy at the higher price will never be able to recoup their investment.