eBay do actually limit the number of times you can pay your seller fees, primarily because even though you may use an instant payment method like PayPal or a credit card, it doesn't always process and update immediately, so your account balance may not always be accurate if you go to make more payments. I was always under the impression it was supposed to be one payment per 24 hour period, and I also believe some sellers have struck a weekly limit as well - I personally make no more than one payment every two days, for an average of 3 payments a week.