Disclaimer: I rarely look at my own demos, I find it's not exactly the same as SC, since you don't hear team comms in the demo, but I'll give a shot at what you should look for when you do. Also, I don't know what rank you are, so some of this may sound a bit obvious, or some of it may not.
In terms of buying at wrong times, you should realize it when you're the only one that bought, and the rest of your team didn't, or the other way around. Don't think it's necessary to look for in the replays.
Aside from what you've said, here's some other things:
- Do you reload immediately after you kill someone?
Don't. Until you're 100% sure you're in a safe place, or your ammo is at 0 You should always expect someone else will come around the corner and try to get a trade kill after you killed their teammate, you don't want them to catch you reloading. You can practice this by playing deathmatch, and avoid reloading until you die or run out of ammo.
- Are you randomly quick-switching your weapon when you shouldn't be?
Sure pros look fancy and quick-switch their weapons all the time, but don't do it when you're about to enter a site, or round a corner. Opponents always pop out at the worst times. Similarly, can also be practiced by playing deathmatch, because opponents come from everywhere.
- Are you running with your rifle/pistol/nade in hand in "safe" areas when you could be maximizing your time by running with your knife out?
I think you know this, but running with knife out is faster than having a weapon/nade. It cuts seconds off reaching certain areas. It's hard to define "safe" areas aside from your spawn in the first few seconds and it always depends on the situation, but as a simplified example: You're on dust2 and it's 1v2, you're terrorist with the bomb on cat and just killed a CT, making it 1v1. You know the last CT is on A site (just from another engagement seconds earlier). You want to run to B site to plant the bomb (there's other metagame stuff and tactics you can do to win the 1v1 but w/e this is simplified), this means that everywhere from cat to lower B to upper B to entrance of B, is essentially safe for you to run as fast as you can towards the bombsite, so you can run with your knife out because you won't even run into the CT until maybe when you enter the B bombsite.
- When taking sites, whether as CT or T, are you taking the sites in coordination with your teammates?
Don't run in one by one and get picked off. Go together.
- Are you making use of your nades before you die?
If you bought nades, make sure you use them. It's situational ofc, and exceptions happen, ie you die after losing a duel etc but try not to die with a full set of nades.
- Kind of related to above, are your nades screwing over your teammates?
Sure you're flashing, but if your teammates are already in the area you're flashing and run into your flashes, well that sucks because you probably just got them all killed. At lower levels (novas and below), nades are more of a detriment than help because people don't know how to use it. You can get by with just purely outaiming your opponents.
These last 2 are probably more useful to pay attention to during the game. =\
- Are you and your teammates constantly being flanked by the opponents?
In MM, you really have to be a very flexible person to do good/decently. Teammates are going to be greedy/selfish etc. For example, CT side, I'd just let them call wherever they want to play, and then fill in the last spot. T side, if you don't have a plan and just want to work picks, try to cover the area that your teammate isn't covering. I'll use dust2 as an example again, for a T side default, you want a setup like
this, 1 person covers upper B tunnel push, 1-2 people covering lower B push, 1-2 watching mid, and 1 person watching long A. The person watching the flanks, long A or upper B, are one of the most important people in that setup. Because if you try any strats that focus on taking the middle/cat area, you do not want to be flanked from behind. This is something you should technically be doing in-game actually. =\
- Are you checking your radar?
Typically I have the radar zoomed all the way out so I can see the entire map. "cl_radar_scale 0.3" This way, you literally see everything that's happening (helps a lot on CT side), and can call/help teammates call because sometimes in MM, people do not talk. Do not over rotate (I guess this can be a separate one too). Try to not rotate unless you see the bomb (when you're playing CT). For T side, if you're on the other side of the map, and you see one of your teammates being overwhelmed by 4+ people, you know it's safer to go to the opposite side he's at because little to no one should be there. The smaller radar takes a while to get used to, but the information it provides is ridiculously helpful, especially if you're used to it from BW/SC2 and always checking minimap. Try not to look too often, especially when you're holding a corner. Opponents love to always peek your angle when you're looking at the radar and get the jump on you =(.
shit this got a bit long =\ and I don't even know if it's any help. Anyways, to finish it up, you should typically just look at all the times you died, and think to yourself, how could I have played this situation better the next time so that I kill the person, instead of them killing me. And then try to remember to use it for the next time.
In the end, it all comes down to playing a lot, and getting that experience.
As for the person to help you, you can try to put your name up in the Player's Club thread, and play consistently with the same people (in the same level), or join a team (again depends on your level). Playing with the same people will tend to critique you a bit more and try to help you improve what you could do better next time in those clutch situations and what not.
edit: Also, try to watch streams of pros when they play MM/ESEA etc and see what their thought decisions are. You can also watch GOTV demos of pros and just stay on their point of view, even when they die. It gives a bit of info on their thought process and see how they react when encountering certain situations. This
youtube channel has some good POVs of pros, or you can even check some of
TL`adreN's videos when he was playing on TST. You see his POV, and also his communications with his team to see his thought process.