The short answer: buy singles on TCGplayer where sellers compete on price, buy sealed at retail from Pokemon Center or big-box stores, and hunt below-market deals locally through Facebook Marketplace, OfferUp, and yard sales. I bought cards through every channel on this page while running a card business, from distributor wholesale down to twenty-dollar yard sale boxes, and each source has a different job. Whatever you buy and wherever you buy it, know the price before you pay: every card's live market value is on its card page here.
Online retail: convenience at market price
TCGplayer for singles, eBay for graded and vintage, Amazon and big-box sites for sealed, and Pokemon Center for new product straight from the source. Expect market price or MSRP, buyer protection, and zero hunting. Retail sometimes surprises you: I have seen booster boxes at Pokemon Center priced well under what marketplaces were asking for the same box. Before paying any sealed price anywhere, check the box against its pull value on the booster box EV tracker; it tells you in one number whether a "deal" is real.
Local marketplaces: where the deals actually are
Facebook Marketplace and OfferUp are where collections surface. Pokemon is one of the biggest media franchises in the world; more people near you have cards in a closet than you would guess, and most of them want one simple cash deal, not a card-by-card negotiation. This is the best risk-adjusted source for anyone buying to resell. Two rules from experience: meet safely (public place, bring someone for bigger deals), and price the collection on live data while you are standing there. The market boards on your phone turn a guess into an offer you can defend.
Card shops, hobby stores, and antique malls
Search "card store near me" and go. Local shops charge closer to market than a yard sale, but they are the community hub: leagues, trade nights, and first crack when a collection walks in the door. Antique malls are the sleeper. Years ago I walked out of one with Fossil and Team Rocket packs at prices nobody would list today. Finds like that are rarer now that the hobby has grown, but old inventory still surfaces in places card people do not look.
Tournaments: trade, connect, and hunt the rare stuff
League challenges, regionals with over a thousand players, and the international championships concentrate collectors like nothing else. Events discourage open selling, so think trading and relationship building. The top end of the hobby lives here too: trophy cards and championship promos that never touch a store shelf. Even as a business buyer, the people I met at events turned into better deals than the events themselves.
Yard sales and thrift stores: the long shots
The hit rate is low and the payoffs are absurd. I have pulled Gold Star cards out of what a seller called bulk, and bought collections for twenty dollars that were worth hundreds. Treat it as a habit, not a plan: check the card bins whenever you are in a Goodwill anyway, and know what the hits look like before you go. Our most valuable cards list is a good mental checklist of what deserves a second look.
Buying as a business: go upstream
If you are sourcing to resell, the endgame is buying closer to the source: distributor accounts for sealed product, standing bulk-buy arrangements, and being the known card buyer in your area so collections come to you. That is a business decision more than a shopping decision, and it usually arrives together with opening a real storefront; the setup path is in the card store guide. Start with the selling guide if you are still deciding whether reselling is for you.