The most popular subreddit for selling baseball cards on Reddit is r/baseballcards. This subreddit has over 160,000 members as of 2022 who are actively buying, selling, and trading cards. Some key points about selling on r/baseballcards:

You’ll need to take clear, well-lit photos of the front and back of each card you are selling. Make sure to include a handwritten note with your username and the date in the photos to prove ownership. Poor quality photos will limit interest in your cards.

Price your cards fairly based on recent eBay sold listings and magazine guide prices like Beckett or PSA/DNA. Overpricing will lead to fewer sales, while underpricing could cost you money. Be flexible within reason.

List the specific cards you have for sale along with individual prices or bundle prices. Provide clear shipping costs, usually $1-$4 depending on the quantity being shipped. Shipping is usually buyer pays unless otherwise stated.

PayPal Goods & Services is highly recommended for payments to protect both buyers and sellers. Clearly list if you accept other payment methods like Venmo or Cash App. Only accept payment once you have dropped cards in the mail.

Communication is key. Respond to any messages within a reasonable timeframe, usually within a day. Provide tracking numbers for shipped orders. Address any issues timely and try to resolve disputes amicably.

Pack cards carefully between cardboard or in toploaders for shipping. Use bubble mailers or rigid envelopes, not regular envelopes which could damage cards. Consider insurance for higher value orders.

Be knowledgeable and honest about card conditions using standard terms like near mint, lightly played, etc. Grading/authenticity disputes can arise and damage reputations.

Another good subreddit specifically for higher end vintage cards is r/baseballcardvintage. It has over 17,000 members. Selling guidelines are similar but focus is on older, rarer rookie cards from the 1950s-1980s usually worth $50+ individually. Organized monthly group breaks also happen where participants can win valuable vintage rookies.

For lower priced individual modern cards or team lots, r/sportscardsell can work well which has 42,000+ members. Similar sale guidelines apply but wider mix of sports included beyond just baseball. Some also have success cross-posting their baseball card sale listings to trading card related subreddits like r/pkmntcgtrades which focuses on Pokemon cards but welcomes other sports/hobbies.

While not exclusive to Reddit, popular third party websites like ComicConnect and Heritage Auctions also allow setup of individual baseball card auction listings that may reach a wider collector audience beyond just Reddit. Their established auction/ consignment sale model provides seller protections similar to major auction houses but with lower fees/commissions on single card prices usually below $1,000 value.

In summary, Reddit provides baseball card sellers reliable communities and platforms to reach active buyers safely through well-established sale guidelines within popular sports card focused subreddits. Just require thorough photodocumentation, fair pricing, communication, and careful shipping to facilitate smooth transactions and build a positive seller reputation overtime.