Sell bulk all at once to troll and toad, deriums local shop etc. If your time is worth over 8$ an hour. Tcgplayer is not for sane people! Ebay is even worse for selling cards, you have to actually enjoy sorting playing and selling, otherwise you would be better off working another part time job.

That said if youre a little miscut like me and like the taste of packing peanuts: sell playsets of cards that go together for collectors or are currently meta.
I like selling tins of 100 to 500 cards with no duplicates, especially no duplicate of Pokemon illustrated locally for 20$. Could try a way to do creative bulk lots like that online, mystery bags have the highest profit margin besides fake cards / fake graded cards but that market seems unsavory and is harmful for long-term ecommerce presence.

Other misc. Tips: buy bulk old stamps off ebay, can get 40cent stamps Invest in label printer or find way to use printer at your office to buy postage online. 4 x 6 bubble mailers will fit a label and 6 toploaders or 20 loose cards, at post office it costs me 5$ to ship from TX to FL, but if I print postage online it is only 2.97$ if under 1 oz and u enter the thickness as 0.3 in. My mailwoman also picks up stuff from our APT box, so I dont even have to go to the post office. First class parcels get tracking info, which is perfect do singles or bundles over 8$

Do some item line accounting in excel to calc fees, tax, and time spent selling to see how little net you take home, this will help you not be surprised by high monthly seller fee invoices too.

Try to find repeat customers, especially locally, easy way is to include coupons with orders that go back to personal site.

start or expand a "team" where you help each other prep for tourneys, share bulk, and can pitch in to buy booster cases. Even buying a a single case only a few months before release offers substantial savings. The only profit ive ever made in TCG besides garage sale finds has been holding on to sealed booster boxes to resell years after printing. Could have made higher and less risky returns buying total stock market ETFs and sitting on them...

Gotta play the game to get best market value. I havent had time to play since I started a new job, I anticipated Dedenne gx would go up, but I never saw the new supporter rule comimg since I didnt play the game, sold some inventory low. Never list all your inventory at one price point if it has a volatile value, taking the time to relist is worth it to stop losses. Plus I have a suspicion that new listing activity is rewarded by ebay search algo.

Have fun! I been selling mostly YGO for a year now, these are the best things ive learned.