Ebay

TL;DR The work between “I own this thing” and “money arrived” is mostly paperwork and printing, and it’s where the hours actually go. Listings get built through the marketplace API, not by driving a browser — structured item specifics as JSON beat fighting a typeahead widget every time. Drafts are created automatically. Publishing is always manual. Publish is a money-moving, buyer-facing, irreversible action; it gets a human every time. Barcode labels render from the inventory record itself: ZPL → PDF → print job to a thermal printer, triggered from the item’s page. No separate label app, no retyping serials. Pack slips print on a schedule, and the job verifies the printer’s page counter moved before marking the slip printed. Optimistic “it printed” flags are how orders ship without paperwork. Anything bulk or financial — repricing, printing five-plus labels — sits behind an explicit confirmation. Cheap friction in exactly the right places. The boring half is the expensive half Sourcing gets all the attention. I’ve written about the funnel that finds lots and the data model that tracks the units. Both were interesting problems. ...

TL;DR You buy a lot. You sell units. That asymmetry is the whole data-modelling problem, and spreadsheets lose to it immediately. The spine is four tables: purchase_lot → inventory_item → listing → sale, with join tables where the relationships are genuinely many-to-many. Cost gets allocated from the lot down to units, and the allocation locks once the first unit sells. Retroactively re-allocating cost after a sale is how you accidentally rewrite last quarter’s profit. A unit walks a status machine: sourced → processing → drafted → listed → sold → shipped → reconciled, with dead and personal as the two honest escape hatches. Selling quantity-N on one listing is N sales, not one sale with a quantity column. I modelled it the wrong way first and spent August unpicking it. Packing several units into one sellable bundle, and splitting a bundle back into units, both need to conserve cost exactly. Getting that wrong is silent and it shows up as fake margin. The mismatch Here’s the transaction that starts everything: I pay $340 for a pallet described as Assorted Networking and Computer Equipment. One line item, one payment, one date. ...

TL;DR Active eBay listings are asking prices, not realized prices—they lie. Use SOLD comps (~90-day history) to anchor your math. Card identity is everything: name + set + edition + variant + grade. “Charizard” is not a price; “Base Set Unlimited Charizard, PSA 9” is. Sell-through velocity (sales per week) predicts how long your capital sits locked up and is the variable most buyers ignore. Expected value = (median sold price × (1 − fee rate)) − shipping − grading. Apply a 30% margin of safety before you bid. Confidence gates protect you: skip lots where you can’t find ≥3 grade-matched comps or where the price spread exceeds ~6× the median. Buying Cards Rewards Discipline I’ve overpaid for Pokémon lots I “knew” were steals. The problem was never the cards—it was that I was bidding on nostalgia and gut feeling, not math. ...