Will you want to choose only one category to begin collecting your sports memorabilia? If so, learn it, research it, and invest in it wisely. Did you know that trading cards have to be graded? It's one of the important facts you would do well to know as a potential collector of authentic, valuable sports items.
Collectibles fraud is a terrible thing for someone to endure. If you've been saving a special item or items for years, waiting on just the right moment to cash in on your investment, and one day you are forced to sell because life threw you a hard ball, you don't want to suffer the devastation of a fake collectible.
Educate yourself, arm yourself with knowledge. Become a collector who doesn't just pretend to know the art of recognizing authentic sports memorabilia.
If you encounter a dealer who doesn't answer your questions to your satisfaction, it's best to follow your instincts and go elsewhere. Dealer confidence is key, but don't confuse confidence with a con's suave evasions.
Find out the worth of your items. It's not fair to you or to the dealer to purchase an item, then discover it's not quite as valuable as you had first thought, and try to hassle the dealer about a return. Do your homework before purchasing. An honest dealer will want you to be happy with your purchase so you can be proud to do business with them and spread their good name to build their reputation. A dishonest dealer just wants you to hurry up and spend your money and won't want to stand behind the authentication of their sports memorabilia. But by the same token, it's unfair to the dealer to go on with an attitude of "If I don't like it, I'll just return it." They want people who are serious about their sports memorabilia just as much as you want authenticity. It's best to leave, think it over, and return than to buy in haste. You may want to return to the same dealer in the future and should build a good interaction with him or her.
Make sure you are clear on how rare an item is. Limited edition does not necessarily mean the same as limited production. Limited edition means one of a kind, limited production means only produced for a short period of time.
A certificate of authenticity does not assure that you're in good hands with a dealer. Neither do the words 'mint condition'. There is such a thing as 'dealer bias'. It's one of the negative aspects of the business. You may later try to return to the same dealer and sell back the card they sold for a high price in 'mint condition' only to find they reduce the worth. They won't want to pay you what they said it was worth, even though they are the ones who set the price originally.
You shouldn't have to allow the dealer to dictate which authentication service you can use. A reputable dealer will be fine with your choice. But do understand that authenticators may not specialize in the item you are having checked. If they don't excel in this area, you may be disappointed.