A month back, I got charged Insertion Fees on over 80 items which had auto-relisted - they were 30-day "Buy It Now" items which were re-listing under the three free relists.

Got it sorted after a Live Chat session with an eBay rep, but I'll guarantee there are countless users who wouldn't notice such things unless the fees were (as in my case) huge. 

As Kooka said - overcharge each user $10 a month, and it's a nice little earner...