Well, that is up to you and what you need and your ethics.
Many of the manuals on eBay are illegal copies of the factory Ford service manuals, but are complete copies.
You can also purchase the factory service manual from Helm. They are the authorized provider for Ford service manuals. Information on these can be found in your owner's manual.
You can also purchase a membership to AlldataDIY which also contains the factory Ford service manual information in a pretty user-friendly format. For the $25 one-year membership, you also get full versions of the Technical Service Bulletins (TSBs) and Service Recalls. If you choose to renew the subscription after the initial year, the renewel fee is $15 I believe.
-Rod