PSA, or Professional Sports Authenticator, is a subdivision of the company, Collectors Universe, that has been grading collectible items since 1991. Since then, PSA has cemented itself as the world's largest trading card authenticator. As one of the most, if not the most, trusted company for grading collectibles, the PSA-certified logo is a staple in the Pokémon TCG community, whether you're looking to buy or to sell.
When you have a special trading card or collectible you want to appraise, you send it to PSA. PSA examines the item closely to look for flaws and imperfections. Based on the quality of the item, PSA gives the item a grade from 1 through 10, 10 meaning it is near perfect. Consumers will be confident your item is accurately priced and assured quality if they see it has been PSA graded. Pokémon cards graded by PSA are also made known to the community. This way, there is always an accurate count of how many of a certain card there are out in the market. PSA, essentially, sets the value for Pokémon cards on the market.
The importance of grading is often overlooked by Pokémon card investors. The difference between a PSA 9 card and PSA 10 could mean hundreds of thousands of dollars. On PSA's website, a PSA 9 first edition Charizard is rated at $32,000, while a PSA 10 version of the same card can be worth over $260,000! This is because even though there is a certain sized pool of first edition Charizard cards from the manufacturer, each grade level is a separate pool. For example, even though there is an unknown number of first edition Charizard cards in existence right now, it is known there are around 121 PSA 10 Charizards, as revealed by PSA. These are cards that have been submitted for grading and received a 10 out of 10 in quality, therefore, as far as we know, there are only 121 PSA 10 Charizards in existence. This number is really low when considering the demand for this card. With a small supply and an ever-growing demand, the price for any one of the 121 Charizards is going to continually increase.
In conclusion, if you want to sell your cards, get them graded at PSA!