Take it how you will, but casual collectors have been willing to pay a little more for graded singles lately and I do get a lot of repeat business from locals when I list duplicates. I can easily sell a 9.5 base for 25-30 bucks on a good day. A PSA 10 Brees I bought for 50, I listed for $100 and was able to agree on 75 with a buyer. So there's profit to be made when I'm lucky, but I might start subscribing to your newsletter as you seem much more educated on the flip side of the hobby.
For me, it's just a lot of trend studying & trying to apply basic 'Investing' principles to the market (not too dissimilar to how you'd treat the Stock Market). IMO, now is a
horrible time to be buying cards (says the guy who picked up a HQ card for $20 raw *cough*) if they are graded or commanding a premium raw. With that said, I use Beckett like it's the last flea market on Earth and would rather buy a card raw & send that in to be graded then buy a graded card unless it's a super personal PC card.
Realistically, your goal should be to make a
minimum 1.5x profit on any card you sell (e.g. the Brees you sold). Doesn't matter if that's selling a $2 card for $3, a $12 card for $18, or a $500 card for $750. Sometimes you'll strike out but overall, that should be your target baseline. Use sites like 130point.com and see what those 9.5 bases are selling for and stay firm on that price. If you know that 9.5 base can sell for $30 on a good day, why not offer it at $50? Worst case scenario, you have to come down to $40 then $30 anyway but the sale takes a little longer. Best case scenario, you sell it at $50 or even $40 and make more of a profit than in the past selling it at $30.
I sold 3 cards on COMC. Purchase price was $35.21 (including a PSA 9). My sales total was $57.50 (slightly undersold my PSA 9) but I made slightly over a 1.6x return on those 3 cards thanks to getting 2x on a couple Silver Prizms. The key is to know what the demand is price wise and aim for that price first. Be willing to negotiate a little but remember: You are
selling this card. There's no written law that you
have to negotiate a price with the buyer. For me personally, if I ever sell cards on Ebay, I'm listing them at a fair market BIN price with no offers. Take it or leave it and I'll gladly wait it out. I'd rather wait 3 months and get the sale at my initial $100 asking price (especially if that card's been selling for say $120) than panic after 2 weeks and settle for $65.
The 2001 PSA 9 Drew Brees (Base Topps RC) 2 years ago was fluctuating between as low as $32 and as high as $90 into January of 2020. The BGS 9 was seeing similar trends. It's dipped a little bit recently but sales are still at $137 - $150 and again, that's a BGS 9... This market is insane. Dan Marino's Rookie PSA 10 was $50 - $75 in 2010 and was pretty consistently between $100 and $130 from 2014 to 2018. Somebody could've easily gotten 2x or even 3x from that card just in the normal market. Even in May, it had basically doubled in price and would've guaranteed a 4x sale if somebody had picked it up for just $50 in 2010 just 10 years later.
I target serial #'d stuff in case the bubble bursts and raw base or even graded base plummets. People are gonna look at serial #'d cards and I'll already be ahead of the curve somewhat in that regard. I know there's only 24 other copies of my Tyreek Hill card or Optic Travis Kelce card in the entire world. I know there's only 4 other Brad Marchand cards in the world. As a bonus, my copy is graded (by GMA but still) giving it a slightly better boost in value as well.
I'd advise you to start keeping a Google Doc or Excel Spreadsheet or even a handwritten log: Player Name - Card - Graded? - Purchase Price/Investment Price (Purchase + Grading Cost) - Sales Price - Profit
Example: Drew Brees - 2001 Topps RC - Y PSA 9 - $75 - $135 - +$60. That will give you a better idea of what you've picked up and at what cost, what you've sold the card(s) at, and how much of a profit you made. You can even throw in a 'Date' header as a reference as well if you sold a Brees at X in 2018 and you're seeing it at Z Price in 2020 then you can expect to sell it at Z Price instead.