Weren’t adverts traditionally what kept an encyclopedia running?
Wikipedia was originally a private project hosted by Jimmy Wales, an internet entrepreneur. When the project became too expensive the idea of accepting advertisements as a source of income came up. Finally Wales decided to pass Wikipedia to the newly found Wikimedia Foundation. Donations make it possible to keep Wikipedia and the other Wikimedia projects the way as they are, without advertisements. The issue, nonetheless, is still hotly discussed at Wikipedia.
In the history of encyclopedias, it was not too unusual to inform the readers about nice things to buy. John Harris wrote in his Lexicon technicum (1704) about mechanical instruments produced by specific instrument makers – most of them were subscribers of his work. Harris did not see anything wrong in that, it was the same to him as quoting from books, of course the best books there were. Isn’t quoting a kind of recommendation as well? 
Even in those times, the 18th century, there were lines that could be crossed. Dennis de Coetlogon used some treatises in his Universal history of the arts and sciences (1740s) to boast about his abilities as a doctor. At the end of the treatise ‘Chirurgery’ he mentions one of his inventions, a ‘vulnerary and styptic tincture’. His modern biographer, Jeff Loveland, calls him a ‘quack’ who mentions his tincture in several treatises and sees more and more diseases it cured. 
Later encyclopedias dropped that kind of business but went on with advertising for themselves. Encyclopedia Britannica in the 20th century spent much more on promotion than on improving content. ‘Executives are ready to use any emotion that will help sell their product’, warned EB critic Harvey Einbinder, especially emotions potential buyers’ had for their children. He quoted from a 1961 advert: ‘How will they measure up against the kids next door?’  This in spite of the fact that EB (and other encyclopedias, usually) is not written for children.
One can expect that the publisher of an encyclopedia praises his work in the preface. In the articles ‘Encyclopedia’ it is common to find at least a mention of the work one has in one’s hands. The last printed edition of the multi-volume Brockhaus-Enzyklopädie (2005/2006) uses the occasion for an indirect self-promotion.
There we read at the end of the article ‘Encyclopedia’ that ‘it must be assumed that the electronic encyclopedias will go on to establish themselves; next to them there will be a continuous interest in printed editions.’ The article also emphasizes that the high quality paper used for Brockhaus-Enzyklopädie will make the volumes endure for more than 400 years. 
I couldn’t help thinking of an old sketch of the German humorist Loriot, about an interview with a salesman of nuclear bomb shelters. To reuse one of the questions: ‘Can’t you imagine that even happy consumers after such a long time are no longer content with their purchase?’
 Jeff Loveland: An Alternative encyclopedia? Dennis de Coetlogon’s Universal history of arts and sciences (1745). Voltaire Foundation, Oxford 2010, p. 163.
 Jeff Loveland: An Alternative encyclopedia? Dennis de Coetlogon’s Universal history of arts and sciences (1745). Voltaire Foundation, Oxford 2010, pp. 157-159.
 Harvey Einbinder: The Myth of the Britannica. MacGibbon & Kee, London 1964 (reprint 1972), p. 269.
 Harvey Einbinder: The Myth of the Britannica. MacGibbon & Kee, London 1964 (reprint 1972), p. 320.
 Enzyklopädie. In: Brockhaus Enzyklopädie in 30 Bänden, 21st edition, vol. 8., pp. 174-180.