One of common strategies if you have a large holding in a company and want to diversify is to sell "half". That said, people also tend to trade around round numbers, so the fact that Marc started trading when the stock crossed $100 might have been part of an earlier plan where "if this ever crosses $100 I'm going to cash some out." I bought some Facebook when it was under $20 but left instructions to sell it when, after fees, it doubled the original investment. So pretty much sold it all at $40 ish.
The point I'm trying to make is that buying and selling stocks usually says more about the investors rate of return goals and less about their opinion of the underlying company.
As a director in a public company, he is very restricted in when and how he sells his stock. In one case I know of that meant placing sell orders 3 - 6 months in advance regardless of the stock performance.
Insiders can file 10b5-1 plans - committing to a sales plan in advance.
"These plans are widely used by officers and directors of public companies to sell stock according to the parameters of the affirmative defense to illegal insider trading available under Rule 10b5-1, which was adopted by the SEC in 2000."
I understand the lack of discretion, I've been an "insider" more times than I would have liked but also look at the link on 10b5-1 plans @jhulla posted. I don't know what Marc did or didn't do, but it could easily have been "sell stock when the net proceeds of the sale will be $100/share or more, up to half my class A holdings.
A perfectly legit 10b5-1 which would have resulted in exactly what we saw here. In February Barron's reported Andreesen's sales as being directed by a 10b5-1 plan: http://www.barrons.com/articles/SB51367578116875004693704580... so he's clearly familar with the process.
Also consider that Peter Thiel sold $100 million worth of stock back in August at $93. A significantly larger size at one price as opposed to Andreessen's multiple sales in the past 2 weeks.
The first block was reported on some news wires on 11/3 when the first SEC Form 4 filing hit, but it wasn't really picked up by more outlets until a few more days of sales (2 more Form 4 filings).
In the same time period he bought 50,145 shares of HP (on 11/2).
Andressen is an exceptional growth investor. I'd expect these funds to be put into new early stage growth opportunities, and I wouldn't be surprised to learn they are outside traditional tech.
The point I'm trying to make is that buying and selling stocks usually says more about the investors rate of return goals and less about their opinion of the underlying company.