The main sports card graders are PSA (Professional Sports Authenticator) and BGS (Becket Grading Services). These companies bring the highest premium for sports cards because their grading system has a low toleration for any defects or minor flaws present on the card. BGS also has BCCG grading which is a bit cheaper and has a higher toleration rate for flaws on cards. For example, a BCCG 10 Mint or Better does not necessarily mean that by BGS standards that it would be a BGS 9 or BGS 9.5. When buying a BCCG 10, one should not pay the equivalent of a BGS 9 or comparable product, the price should be a little bit lower. At one time, I saw a bunch of 1994 SP Alex Rodriguez rookie cards, graded BCCG 10, selling for prices even higher than BGS 9's. But, this was apparently just the market not understanding the grading tolerations. Now, prices are comparable to that of BGS 8.5's more than BGS 9's.
PSA and BGS do bring the highest premium when selling your sports cards but I find it much better to buy a card graded by SGC (Sportscard Guarantee). Their grading toleration is comparable, if not lower, than PSA and BGS. This fact can be seen in the population reports on their website. These are the amount of a specific grade compared to the total amount of cards submitted to them.
Tips on buying: If you are buying baseball cards for profit, try and buy high quality cards only because these are the ones that increase dramatically with a fluctuating market. Usually the best time to buy sports cards is when that particular sport is off season. For example, you may want to buy an Alex Rodriguez RC a couple months after October and before the beginning of the pre-season. Another good trick maybe buying quality SGC 98 and GAI 9.5 graded cards and sending them in for re-grading through PSA or BGS.
If you are buying baseball cards for long term, then you can go ahead and buy just about any card on e-bay, be they graded or not, because part of the fun sometimes is actually sending in your cards for grading and waiting for the results to come back in the mail.
There is also another grading company which is comparable to PSA, BGS and SGC which is called GAI (Global Authentication). These cards sell comparable to SGC graded cards.
Tips on selling: I recommend to sell your sports cards according to what a player is doing during the season. If a pitcher does a no-hitter, doing a 1-day sale after the fact is not a bad idea because that athlete will be searched for alot during the next few days and you need take advantage of that. I also recommend not to sell any expensive cards during the off season, you will lose money if it is auction based. The best way to sell during the off season is doing a buy it now sale. Not many of that player's cards will be on for auction and people will be more willing to buy at a fixed price when this is true.
Ranking of price of graded cards
1. BGS 10
2. BGS 9.5
3. PSA 10
4. SGC 98 and GAI 9.5
5. BGS 9
6. PSA 9
7. SGC 96 and GAI 9
I hope that this guide was useful and I will be writing more every chance I get. Thank you for reading!