It doesn't matter if it's a star player, error card, variation, whatever...
...the card's condition will still be key.
Just like any other type of collector, baseball card collectors want to own quality items.
Buyers and sellers should focus on several key factors that professional graders look at such as:
Centering - From top to bottom and left to right, how well is the card centered? Centering is probably the biggest concern for most collectors. On the front side, if a card exhibits no worse than 55/45 to 60/40 centering on the front and 75/25 on the back then it is usually considered a well-centered card.
Below you can see clear differences in centering between these two 1973 Topps Nolan Ryan cards.
The one on the left exhibits near-perfect centering while the one on the right favors the top and left parts of the card.
Corners - You want to see sharp corners. Ideally "razor sharp" as many people in the hobby like to call them. Round corners are signs of heavy use and are considered eye soars.
Here is a look at a gem mint copy of Carl Yastrzemski's 1967 Topps #355 issue alongside close-ups of each of the four sharp corners.
Edges - Nice, clean edges go a long way to help a card's eye appeal. Sometimes you'll see vintage cards that were poorly cut and don't have nice clean edges. Professional graders (and collectors) will make exceptions for cards with known cut issues. But when possible, you want to see edges free of chipping and notches.
Both of the Jackie Robinson rookies below were graded PSA 9 Mint condition and exhibit very nice edges.
However, if you look closely at the 1949 Bowman, you can see the left-hand edges are not as smooth. That set is well-known to have rough cut issues so professional graders take that into consideration.
Surface - One of the easiest ways to ruin a card's value is if it has a crease on it. Other issues like indentation, marking, scratching, staining and loss of gloss can significantly reduce a card's value.
When you look at these two examples of Joe Dimaggio's 1941 Play Ball baseball card, you can quickly see differences in surface quality.
The one on the left features rich coloring, no creasing, no chipping and has strong eye appeal.
But, the one on the right shows fading, a bit of staining, creasing and paper loss.
You can always refer to cards listed for sale online as a reference only. See what a Gem Mint 10 or NM-MT 8 looks like by finding current examples listed for sale.
But please be careful and remember those are only references.
It's just too tough to notice some condition issues, especially surface and gloss issues, by looking at your card with the naked eye and comparing it to a professionally graded copy online.
Remember: Professional graders use high-tech optical equipment when reviewing them and grading them. They can see errors you can't.