I advertise sales of sports memorabelia items from my website sites. I use web links, banners and graphics and have not had any probems in 12 books so far.
How you finance your production is not a concern for Apple, remember not all people use iBA to make books, some pay others to do them and deliver to the iBookstore.
Better to keep any "adverts" to the end pages of the book rather than infiltrate them into the main body..is my opinion. Or you could use a Sponsors page to acknowledge them.
"iBooks also has a very simple linking architecture that enables you to link directly to books within the Store.
This could be used for marketing from your own website or online advertisements, or it could be used within
an EPUB to direct a customer to other related titles at the end of a book." - iBooks Asset Guide 5.1 Rev 1
I remember reading in one of apple's guides, can't seem to find it now, that you can't link to companies that compete directly with Apple (ie. Amazon.com)
Bringing your book ideas to the eBook market
Specializing in fixed-layout eBooks
I have never tried linking to "other online publishers" - but I do recall some threads where some that did either had a ticket to remove certain web links - or they were not there when the book was in the store.
Apple does protect its products and iTunes stores is a product which the allow use of by those wishing to upload content for sale or free distribution.
Apple are said to block certain names/words such a Jayle Brake or Ham -a Zon. There is some truth to some and myth to others... as your amazon.com proves!
The fact is that iBA allows the creation of web links, - so why not not bung whatever you want in the pages and await an outcome!
A ticket to resolve the issue will not take long to amend and re-deliver.