Early game is an important part of CnC3, since you not only have to decide what you are going to do, but also set your base up as quickly as possible. A very good way to do this is to sell off certain structures, which can give multiple benefits.
First of all, remember that when selling a structure, you can get up to half of the cost of that structure back (depending on how damaged the structure is), which will give you a slight boost in cash, and early on you want all the money you can get. Almost every structure in the game will also give you some infantry when sold, and these infantry can vary from Riflemen to Zone Troopers.
Selling redundant structures
Sometimes early game you will build a structure you just need to accomplish one task early game, and after that it becomes worthless. The most common example of this is the Emissary/Surveyor/Explorer, which is used to get a couple of expansion Refineries and then does nothing. By selling the Outpost once a Refinery is placed, you get $750 back immediately.
The Outpost won't help any more...
...so sell it for $750.
Speaking of Refineries, you can sell a couple of Refineries as well if you feel you don’t need all of them (and you honestly don’t, although the rate of collection can be slightly higher if you keep both Refineries early game). You’ll only get a maximum of $300 back, but you get an extra Rifleman squad (or Buzzers if you’re Scrin), which might be useful for scouting or killing enemy scouts with.
Another example of this technique early game is the Secret Shrine. You need this to train Shadow Teams for the highly effective Shadow Team rush, but it offers no other useful features early game. Therefore once you have trained all the Shadows you want, you can sell the Secret Shrine: not only does this get you $750 cash back, but it also gives you an extra Shadow Team to attack with. Very useful in bolstering your attack and your early cash flow at the same time.
Sell your Secret Shrine for an extra Shadow Team.
Selling defensive structures
Defensive structures will cost you some extra cash early on, but they have the huge advantage of being on a separate build queue to the rest of your structures, so you can build up your base uninterrupted while placing a couple of defensive structures. Once you place a defensive structure, you can sell it to get some extra infantry, sometimes at a lower cost than normal. Another good thing about this is that you can effectively train rifle infantry without a Barracks early game.
Probably the best structures for this job are the Tiberium Silos for GDI/Nod (when going for a fast Warfac build) and the Buzzer Hive as Scrin. Selling a Tiberium Silo effectively gets you a $250 Riflemen squad, and the Buzzer Hive gives you 3 Buzzers for $300 when sold. Scrin definitely have the best deal here, but GDI/Nod Silo selling can be sufficient for getting an idea of any possible early strikes against your base, since you will have no other way of scouting with a fast Warfac build.
Selling can get you extra scouts too!