Should you sell a bulk lot whole or split it? The math
Short answer: list the lines that net more than a bulk sale after fees, shipping and your time, sell the rest on in bulk, and compare that total with the whole-lot offer. Listing everything is usually worse than listing the right things, and sometimes worse than the offer.
A worked example
A lot holds six Pyrex bowls and ten cases of mason jars. Marketplace fee is 13%, unsold or bulk units fetch 20% of price, and your time is worth $15 an hour. A buyer offers $60 for everything.
Pyrex bowls: 6 at $28, 85% sell-through, $9 shipping, 12 minutes each. Listed they net $65.38; sold in bulk, $33.60. List them.
Mason jars: 10 at $4, 95% sell-through, $6 shipping, 3 minutes each. Listed they net −$31.04, because shipping and time exceed the sale; in bulk they bring $8.00. Sell them on.
The best mix is $65.38 + $8.00 = $73.38, which beats the $60 offer by $13.38. Listing everything nets only $34.34, which is $25.66 less than the offer. The Lot Split Calculator runs this on any list, with the stress test and the break-even rate.
How the answer is found
Each line gets two values: listed (sold units net of fee and shipping, unsold units at the bulk percentage, less your hours) and dumped (the bulk percentage of price, no time).
Lines do not interact, so the best mix is the larger value on every line. The tests check this against every possible list-or-bulk combination.
The total is compared with the offer, re-run with lower sell-through, and re-solved across hourly rates to find where the offer starts to win.
Know the break-even hour
The rate at which your time makes the offer and the split tie is the number to argue with. If an hour of your time is worth more than that, take the offer and move on; below it, splitting pays you for the work.
Questions
Should I always list every item?
No. Each line should be listed only if it nets more than selling it on in bulk, after the marketplace fee, shipping and the value of your time. Low-priced, bulky or slow-selling lines usually fail that test.
How do I value my own time?
Use what you would earn doing something else with the hour, or the rate you would pay someone to do the listing. The calculator also shows the hourly rate at which the whole-lot offer ties your best split.
What if I do not know the sell-through?
Use a conservative guess and check the stress test: the tool re-runs the answer with sell-through cut to a lower share of your estimate, so you can see whether the decision survives a bad month.