Hi, Peter. I have been record collecting for 30 years, and the first thing you'll need is a way to clean the used records, because many will be very playable under the coating of dirt that is commonly on them. When people are selling records, they generally don't have them cleaned for you. You have to do it yourself. This will elevate quite a number of noisy LPs into the playable category.
Now to judge them, you need to be able to look through the dirt at the actual surface of the record. Generally, if the surface is shiny, that is a good sign. Very dull surfaces typically indicate high wear. But not always. Slight marks that could have been caused by the paper sleeve are not a concern for playability. Even slight scratches that are not deep, may not even be a cause for concern, although they may possibly be slightly heard. The method that I have determined about the playability of scratches(from visual inspection) is that if the scratch goes across the blank band between tracks, and is not making a mark across the blank band, then it is likely to be ok. If it is deep enough to make a mark across the blank bands, then it is probably not ok. This is not to say that you may not hear the "ticks" as it passes the scratched areas, but they will probably not be any worse than a dust "pop" or other random noise that you will get on a used LP. Other damage such a gouges or divots, are much more likely to make the record unplayable, at least on the songs where they appear. I have bought many albums with what appears to be disqualifying damage, because the damage is a gouge, and it is not on any of the songs that I want to play on that album. If all the songs I want from that album, are not affected, I'll buy it anyway, if it is a low price.
Now, when you get the album home, and play it, even after cleaning, you can have a record that sounds like crap, because sometimes you just cannot see visually, the wear that has occurred. This happens fairly often, and is just part of the deal when you buy used records. If it has been played on a poor TT,then it may have some of the highs shaved off. During those passages, you'll just hear a "cchhhhh" where the singer is belting out a high note, or something. That record is history. Nothing can be done about that. I have bought records that looked perfect, that were afflicted with that problem. That is just the luck of the draw.
Nothing is automatically rejected until you look at it closely. I have bought many records which had no inner sleeve.
Now alot of people might reject some records that I would keep, because I can clean them, and I have a very good TT and phono section that keeps noise to a minimum, and I have been listening to LPs all my life, and I have learned to not be overly sensitive to certain kinds of noise. Very finicky people should only buy new records, because they will be too finicky to accept any flaw that might appear on a used record. And used records will have flaws. You learn to be more accepting of this, when you are trying to get an LP that has been out of print for 40 years, and you have only seen 1 copy in your life, and it is in your hands, and it looks like it might have some wear. It is better to have some kind of copy, just to have the music, than it is to reject that copy, and you may never see that album again. This has happened to me a number of times.
Oh, and regarding warps, a very smooth long warp, will be more likely playable, than a short peaky warp. If it looks like a "ripple" then leave it alone. Non-flat records with gradual warps can sometimes sit flat on the TT if you have a record clamp. Dish warps are bad, and will usually make at least one side of the record unplayable, at least for the outer track or two.
Now to judge them, you need to be able to look through the dirt at the actual surface of the record. Generally, if the surface is shiny, that is a good sign. Very dull surfaces typically indicate high wear. But not always. Slight marks that could have been caused by the paper sleeve are not a concern for playability. Even slight scratches that are not deep, may not even be a cause for concern, although they may possibly be slightly heard. The method that I have determined about the playability of scratches(from visual inspection) is that if the scratch goes across the blank band between tracks, and is not making a mark across the blank band, then it is likely to be ok. If it is deep enough to make a mark across the blank bands, then it is probably not ok. This is not to say that you may not hear the "ticks" as it passes the scratched areas, but they will probably not be any worse than a dust "pop" or other random noise that you will get on a used LP. Other damage such a gouges or divots, are much more likely to make the record unplayable, at least on the songs where they appear. I have bought many albums with what appears to be disqualifying damage, because the damage is a gouge, and it is not on any of the songs that I want to play on that album. If all the songs I want from that album, are not affected, I'll buy it anyway, if it is a low price.
Now, when you get the album home, and play it, even after cleaning, you can have a record that sounds like crap, because sometimes you just cannot see visually, the wear that has occurred. This happens fairly often, and is just part of the deal when you buy used records. If it has been played on a poor TT,then it may have some of the highs shaved off. During those passages, you'll just hear a "cchhhhh" where the singer is belting out a high note, or something. That record is history. Nothing can be done about that. I have bought records that looked perfect, that were afflicted with that problem. That is just the luck of the draw.
Nothing is automatically rejected until you look at it closely. I have bought many records which had no inner sleeve.
Now alot of people might reject some records that I would keep, because I can clean them, and I have a very good TT and phono section that keeps noise to a minimum, and I have been listening to LPs all my life, and I have learned to not be overly sensitive to certain kinds of noise. Very finicky people should only buy new records, because they will be too finicky to accept any flaw that might appear on a used record. And used records will have flaws. You learn to be more accepting of this, when you are trying to get an LP that has been out of print for 40 years, and you have only seen 1 copy in your life, and it is in your hands, and it looks like it might have some wear. It is better to have some kind of copy, just to have the music, than it is to reject that copy, and you may never see that album again. This has happened to me a number of times.
Oh, and regarding warps, a very smooth long warp, will be more likely playable, than a short peaky warp. If it looks like a "ripple" then leave it alone. Non-flat records with gradual warps can sometimes sit flat on the TT if you have a record clamp. Dish warps are bad, and will usually make at least one side of the record unplayable, at least for the outer track or two.