Every round you get money for doing something, i could write down every little thing, but someone else has already done that. Thanks TheWarOwl!

Should you buy on the pistol round?
Big freakin' YES.
Though you can't do much with 800, your teammates can help. winning the pistol round will net you 3 more rounds, so here's what you do.

VEST - $650
True, if you buy this, you'll only have enough money for a decoy, but the vest is extremely important. It's going to soak up a majority of the damage you take.

Pistol: $300-$700
Pistols are the good thing to have on this round, depending on which one you pick, it could make or break the round, get some practice in and see which one suits you best. P250s and Five-SeveNs are the best choice, though.

Nades - $50, $200, $300
Decoys aren't very useful, but serve as a false flash. Flashes themselves are always useful. and smokes are pricey, HE Grenades can do a lot of damage against unarmored rushes.

Don't forget to experiment, buy for your friends, try new strategies.

"What's the point of buying these shoddy weapons?" cries the silver 2 at the back of the room. "I have enough for a fullbuy!"
Ecobuying comes directly after the pistol round, but only if you win it.
The point of an Ecobuy is to gain:
https://www.youtube.com/watch?v=ON-7v4qnHP8
In other words...
SMG KILL AWARD: $600
SHOTGUN KILL AWARD: $900
What should you buy on an ecobuy? Allow me to elaborate.

NOVA
It's cheap ($1200) and 2 kills with it will pay for itself, with interest. it's a good ambush weapon and the $900 kill award is good.

MAG-7
For that point where you need a little more oomph in your shotty. It's the best of them and it's price is low. ($1800) 2 Kills will pay for itself.

SAWED-OFF
Same cost as the nova, but with a higher firing speed and Terrorist exclusive.
The downside is that it's more suited to defense, and Ts attack.

Avoid the XM1014, too expensive and too innacurate.

MP9/MAC-10
Now we get to the SMGs. $600 kill award, but more accurate and faster firing.
The MP9 and MAC-10 are good for Ecobuys, costing $1250 and $1050 each.

MP7
More accurate, More damage, More armor penetration, $1700, Need i say more?

Don't buy the P90, it's a forcebuy weapon, has a completely random spread, and has no kill bonus.

What happens if you fullbuy this round?
If you fullbuy, you're giving saving players the advantage, most eco weapons are crap against armor, so if a saving player takes someone out, the weapon they get won't do much.
But if that ONE GUY gets an M4/AK, suppose a saver buys a zeus, and ambushes them, now they have a fullbuy weapon, and stand a chance against you.

If you win the eco round, have another go at it, they'll most likely be saving for another round.

Well done, you lost the pistol round, it's not the end of the world, though. Buy a P250, or whatever pistol you're best with, get in a spot and prepare to ambush whoever runs by. Your goal is to pick them, grab their weapon, and run. Doing so does ecological damage to the enemy.

If you fullbought on the Eco round, you just made an Anti-Ecoer happy. And your team mad. Because the enemy now has the advantage.

You have no money. You lost the pistol round. You just can't afford anything in general, what do you do?
Simple: you forcebuy and charge in and die.
Nope. you save.
Save rounds are just that: You buy nothing. Save your money, Get the game when it goes on sale instead of spending all your money on it now, Or, at the most, buy a P250.
The thing about save rounds is that you're almost supposed to lose, your goal is to get ambush picks, or just to get money from round loss. Either way, it's still possible to pull a win out of a save...
http://youtu.be/RimNm7_Z6uE

What is Forcebuying? Well, let's put it this way, your economy is screwed. You have $3750 while your teammates have enough to fullbuy, alternatively, the enemy is nearing matchpoint and you can't stop the momentum. Here's what you do.
Step 1: Buy armor
Step 2: Buy the best weapon your money can afford. Mag-7s, FAMASes, Galils, and if the situation calls for it, maybe a P90.
Step 3: Fill in the blank spots with Nades, defusers, ETC.
Step 4: Do your best, get a better weapon if one drops. and DON'T THROW.

Alright hotshot. You just won the Pistol Round, and the Eco round, or you've saved enough to buy yourself anything you need to take out the enemy team. But when exactly should you buy? And what should you buy?

WHEN TO FULLBUY
Look around you, check your teammate's cash values. If you've been properly balancing, they should be near-identical. If you have an uneven economy, consider buying a weapon for the broke player.
Here's some simple math to help figure out when to buy.

CT Side
WEAPONS CT SIDE ARE VERY EXPENSIVE. Most maps are CT-Sided, to balance this.
M4A4/A1-S: $3100
Armor: $1000
Defuse Kit: $400
Nades: $200, $300, $300 (You should most likely have a Flash or two, and a smoke. HE Nades if you have extra pistol round money, probably Five-SeveN + HE Nade)
Total: $3100 + $1000 + $400 + $500-$1000 = $5000+

_________________________________________________

T Side
AK-47: $2700
Armor: $1000
Nades: $200, $300, $300 (Nades are much more useful T-Side, Flashing before entering a site, Smoking off rotate points, ETC.)
Total: $2700 + $1000 + $500-$1000 = $4200+

_________________________________________________

AWPer Buys
AWPs are another story, just take out the Rifle of a fullbuy and replace it with an AWP.
AWP: $4750
Sidearm: $300-$700
Armor: $1000
Nades: $500-$1000
Total: $4750 + $1000 + $500 (Rounded for pistols) + $500-$1000 = $6750+

Alternatively, If you have lots of money in reserve, buy an AWP for your teammates. And grab one at the end of a round if you can.

It's the final round, Money doesn't matter anymore, Spend all your money, Buy yourself an AUG and a Five-SeveN, Nades, whatever you want, spend all your money and don't worry.

•Get in a team. Think like a hivemind. Know when to buy, be sure to co-ordinate your buys with your teammates beforehand.

•Be sure to keep the economy even, buy weapons for broke players if you have excess. If the team is saving and you have much more money than them consider buying armor and a pistol.

•If you can't clutch and you can't afford to buy next round, consider "Saving", just get away from the action and make sure no one finds you. There are exceptions.
DO SAVE:
AWPs
Rifles
DON'T SAVE:
Starting Pistols
SMGs
Scouts
At match point/halftime
70% of the time on T-Side. Saving on T-Side gets you a fat stack of $0 at the end of the round.

•Don't buy LMGs. Just don't.

And most importantly...

•Be sure to keep the economy even.