The only reference I've found to buying limits is this;
"We've set buyer activity limits in the eBay system to encourage safe and responsible buying. If you come up against activity limits, the best way to remove them is to put a valid credit card on file as additional verification. You may be charged an authorisation charge of £1, but this will be refunded back to your card after seven to ten days.
Here's how to place a credit card on file:
1. Go to Site Map at the bottom right-hand side of the eBay homepage > My eBay - Selling > Place or Update Credit Card on File and sign in.
2. Enter your information on our secure form and click 'Continue'.
If you'd rather not put your details on file, we recommend you continue to build up positive feedback before selling or bidding on high-priced items."
That's taken from one of their customer support pages.
Geoff