Just checked DMD diamond and that coin doesn't look healthy at all. 24h trade is only 160k which is below 10k coins. So 24h trade is less than one masternode! Seem's like really unstable coin which could be below 1$ tomorrow. Selling masternode would dump the coin down heavily. put that in perspective it would be same as dumping 726854 bitcoins to market. Although with bitcoin there would be actually buyers in the market.
Yes, DMD trade volume is low (which is why the Dev is actually urging people to sell some, which in itself is a pretty strong signal), but my point is DMD is more than four years old and had slow steady growth for most of the time, so quite the opposite of an unstable coin. The Masternode number has also been growing since the introduction of Masternodes, so at least some people seem to be willing to invest quite a lot.
I also disagree that investing in a MN and selling a MN for the same price is the same, if you hold a MN for a long time you know rewards work, are stable etc., and you are also psychologically more invested in the coin, but at the end of the day everyone has their own investment strategy...