The same thing happened to me - I sell the same now per month with 250 items as I did with 50 items. Except for this month when ive sold almost nothing.

Several Ebay employees, including the CEO, have admitted hidden seller limits and throttling. In their terms and conditions it also states that they can, and do, put limits on accounts when they please and theres nothing to make them actually tell the account holder these limits are in place (they probably arent per account, rather per group of sellers that fit a predetermined set of values?).
Official statements from Ebay also confirm they have seperate contracts with larger companies (and of course they do anyway, all businesses have several contract levels for different parties, it would be bizarre if they didnt).

Amazed people still argue whether or not this exists when its been stated and its in black and white. Its like they think they need to understand why for it to exist. Strange.

In regard to those saying theyve had an awesome month, theres probably many who have had a great month, they would be picking up the buyers from those who are being throttled. The traffic is still here, its just being diverted away from some sellers.