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The FDA maintains a very nice database of all drugs licensed in the United States. It is updated every monday, and they even document the file format. AWESOME.
http://www.fda.gov/Drugs/InformationOnDrugs/ucm142438.htm
DrugBank also has a very good database of drug information, complete with information on what each drug is used to treat. The FDA database is more official, but it lacks this contextual information. The DrugBank data is legit, even if the website does look janky.
http://www.drugbank.ca/downloads
Each state has its own pharmacy registration & verification system; I reached out to the NABP to see if there was any service that could simplify it to verify pharmacies for all 50 states:
We will be using PostgreSQL for this project because all of our data will be very well formatted, and we can make SQL ridiculously fast with this kind of data. Also because postgres has fantastic support for high availability and replication. Unfortunately this precludes us growing into AWS RDS, but frankly RDS is just a fancy wrapper around EC2 instances anyway.
http://www.postgresql.org/docs/9.1/static/high-availability.html
Paypal: https://developer.paypal.com/
Amazon FPS : http://aws.amazon.com/fps/#highlights1
Google Wallet : https://developers.google.com/commerce/wallet/