How much to pay for a thrift find you want to resell
Short answer: pay no more than the price at which your expected profit, after everything it costs you to sell the item, is still above your bar. That walk-away price can be worked out exactly from the resale price, the marketplace fee, shipping, your time, how long your cash is tied up and how likely the item is to sell. The picker does it for a whole cart at once.
Everything below is arithmetic on invented numbers. The resale prices, sell chances and fees are illustrative, not prices, not a price guide and not a forecast. Replace them with your own.
The arithmetic
If the item sells, you keep the resale price less the marketplace percentage fee and fixed fee, less shipping and supplies.
If it does not sell at your price, you recover some share of its value by marking it down or wholesaling it (30 percent in the worked list), or nothing.
Weight those two outcomes by your sell chance. That is the expected revenue.
Subtract what you pay, the value of your hands-on time, and the cost of the cash while the item sits. What is left is the expected profit.
Profit is a straight line in the sell chance, so the chance at which it is exactly zero is one division. The walk-away price is also one division, and it moves up if you lower your required return.
A worked list
Fourteen finds across clothing, cookware, electronics and collectibles. Defaults: 13 percent plus $0.30 marketplace fee, $0.50 supplies, $15 an hour for 20 minutes an item unless the row says otherwise, a 20 percent annual cost of cash, 30 percent recovered if unsold.
With no caps the picker buys 10 of 14 for $160.00, expected profit $244.16 (about $66.89 per hands-on hour). Four items lose on these numbers.
The console lot asks $30.00 and is worth paying up to $80.12. It breaks even at a 3.8 percent sell chance, but still has a 30 percent chance of ending at -$2.92 or worse.
The portable cassette player is the near-miss: expected -$0.02, worth paying up to $19.99 against a $20.00 ask. A one-dollar negotiation turns it into a buy.
The cookbook set asks $15.00 and expects -$13.15; it is only worth $2.13. Cheap heavy media is where flipping money goes to die.
With $80 of cash and two hands-on hours the exact best set is 5 items for $80.00 and 1.65 hours, expected $152.01. Ranking by return on cash and stopping at the first cap gives $145.02, so the best set is $6.99 better.
Reading the downside
An expected profit hides the bad outcomes. For every item the picker also prints your chance of ending below zero and the worst case. A buy whose worst case is a couple of dollars is a different bet from one that can lose the whole ask.
What to feed it
Resale prices from sold listings you trust, not asking prices.
An honest sell chance within the days you will wait. Be conservative: the picker multiplies exactly what you give it.
The marketplace's current fee percent and fixed fee, your real shipping, and what an hour of your time is worth to you.
A marketplace's own rules about what may be sold come first, and so does checking that an item is genuine and safe to resell.
Questions
How much should I pay for something I want to resell?
No more than the walk-away price: expected resale after the marketplace fee, shipping and supplies, less the value of your time and the cost of tying up cash, scaled by the chance it sells. On the worked list, a $140 console lot with a 70 percent sell chance is worth paying up to $80.12 for, though it asked $30.00.
What sell-through rate do I need for a thrift flip to be worth it?
It depends on the item, but it is solved exactly: profit is a straight line in the sell chance, so the break-even chance is a single division. The $140 console lot needs only about a 3.8 percent chance of selling because even the markdown recovery nearly covers its $30.00 ask; a $22 cookbook set that asks $15.00 does not reach break-even at any chance.
Is my time really part of the cost?
If you value it at anything above zero, yes. A twenty-minute listing at $15 an hour is $5.00 an item, which is the whole margin on cheap finds. The picker lets you set your own hourly value and per-item minutes.
Why does a cap on cash or hours change the answer?
Because ranking by return and stopping is not the best use of a fixed budget. With an $80 budget and two hands-on hours, the exact best set earns $6.99 more than ranking by return on cash.