I don't what it's like in Malaysia, but over here in the U.S. a lot of shops are selling common pressings of common classic rock albums in less than VG+ condition for inflated prices. The shops do this because more noobs are jumping into the vinyl trend every day. My best advice is to avoid spending too much on used records when you are starting out. That way if you make a mistake on an album that looked good under the shop's dim light or whatever but turned out to be a lemon you won't be out a lot of $. I would try to set a limit under $10 per LP if you can for used vinyl, at least until you get more familiar with some of the inspection techniques that have been discussed here. It's also not a bad idea to bring a small flashlight with you to inspect vinyl in shops. As convenient as online buying is, the best way to buy used records is still in person at a record store, IME/IMHO. Oh, and get to know the shop's reputation for returns. If you get a bum record with a repeating skip in it or something, are they are going to take it back no questions asked, or are they going to hem and haw and give you static about it, refuse a return/exchange,etc.? I've learned to avoid shops that do the latter.
RE: Discogs, it can be a crapshoot w/r/t grading. Overgrading is general is rampant on Discogs. So if something is listed as NM-, it might be VG+ or worse. If a record is listed as VG+, it might be more like VG or even G+ and so on. The seller rating system is helpful but there are ways to game that and like I said, more new people are getting into this hobby every day and they might not be keen on giving a seller a bad rating even when they deserve it. My tactic is to ask questions from sellers and only buy 1 or 2 records if the seller is new to me, usually inexpensive records. If the seller won't answer basic questions about an item's condition or if it is the correct edition (lots of sellers listing stuff wrong as well) I simply move on to a different seller. Where people often get burned is ordering a truckload of LPs from a seller they're unfamiliar with because the prices are low and they want to combine shipping to save money.