I think a good place to start would be with the item list the customers gave you. You should try to reproduce the issue yourself, you can always refund a test transaction that you make. It is important that we know the exact circumstances of the error. I'm not aware of any item number limitation in either eCart or with the Authorize.net gateway.
I have seen an issue with Authorize.net when an items name contains more than a certain number of characters. You could test for this by trying to purchase the item with the largest number of characters in the name. If you can make a purchase of that item then it is probably not the issue.