Mahomes cards have been on fire most of the year and they will be peaking right before the Super Bowl.
@Codeine Fiend Dream Team gave a lot of advice. I'll piggy back and lay down my two cents as well.
I would say Mahomes is second in value to every NFL player ever at the moment behind Brady. He's been incredible so far but let that sink in. His shyt goes for more than Manning, a guy who won five MVPs, a couple Super Bowls and had a bunch of passing records when he retired. So if you're buying into Mahomes right now, his prices have basically factored in multiple MVPs, Super Bowls and other records.
Now, maybe he'll accomplish all of that and go down as the GOAT or top three. But there's a bar set for him now and if his play slips at all, those prices are gonna come down accordingly and people will move onto the next hot thing. Plenty of guys were worth a ton but fell off due to injury or play and their cards carry minimal weight. I think he's the real deal and will continue his level of play or close to it but keep that in mind if you decide to buy some of his cards.
Maybe you believe in Kyler Murray. His cards are fairly reasonable right now. If he breaks out next year, his prices will explode too.
If you are buying to sell later, I would always recommend buying graded cards, particularly high grade. Know that PSA 10 Gem Mint cards carry a premium over BGS 9.5 Gem Mint cards. BGS 10s carry a premium over both but personally, I think it's a waste of money to pay that premium. Stick with a PSA 10 for less.
There has been a trimming scandal boiling over in the past couple years so even if the card is graded, do some homework on it beforehand, especially for numbered em rds.
WorthPoint | Antiques, Art and Collectibles - What's it Worth? | WorthPoint and
Market Price Research - PWCC (will need a free account for this one) are your best resources for research.
For high end RCs for both football and basketball, the National Treasures RPAs are always the most coveted rookie card now. That's as high end as it gets but there's also Flawless, Impeccable and Immaculate as well.
There's more but I can't think straight right now. I can answer any other questions you have later.