I didn't see this mentioned and I think it's very important to address.
The type of listing is a HUGE variable here.
eBay allows 250 'free listings'/'insertions' per month for sellers with no store.
Assuming a seller has 250 TOTAL listings created...
If your 250 listings are "BUY IT NOW (BIN) - Good 'til Cancelled", these 250 BIN listings would all relist/renew once per month. That's 250 listings per month and you aren't going over the promotional 250 free listing limit. $0 additional fees. All is good, all is peachy.
BUT, if your 250 listings are "Auction" style, set to "auto-relist", then the listing/insertion number and associated fees WILL creep up on you.
250 "10 Day Auction" listings would relist roughly 3 times per month, resulting in 750 total listings, which is 500 OVER the free 250.
250 "7 Day Auction" listings would relist roughly 4 times per month, resulting in 1000 total listings, which is 750 OVER the free 250.
250 "5 Day Auction" listings would relist roughly 6 times per month, resulting in 1500 total listings, which is 1250 OVER the free 250.
250 "3 Day Auction" listings would relist roughly 10 times per month, resulting in 2500 total listings, which is 2250 OVER the free 250. (Sometimes even 11 times, 2500 over 250!)
Since eBay is currently charging $0.35 per listing OVER the 250 free insertions, your costs could be staggering:
$0.35 x 500 = $175 additional fees per month
$0.35 x 750 = $262 additional fees per month
$0.35 x 1250 = $437 additional fees per month
$0.35 x 2250 = $787 additional fees per month
$0.35 x 2500 = $875 additional fees per month
Something to consider - a 'listing' is not the same as an 'item'. You may have 1 ITEM that you are listing and relisting until it sells, but each listing is +1 to your LISTING / INSERTION count.
Another note, and this has been implied, but it IS confusing, when eBay indicates your number of remaining monthly Promotional Free Listings (Used/Left: 0/250), they do NOT consider or calculate the active listings that are setup to auto renew in the current month. So essentially, you'll need to be aware of how many will relist and manually calculate this yourself.
I sincerely hope this helps someone!