Modern English is partially the product of an unnatural grafting of French onto Old English. It is for this reason that we often find two words for nearly the same thing. Thus, we call the animal a cow but the food beef. The barnyard term is Old English, cu, while the table term is Old French, buef. This should be no surprise for after the events of 1066 the hands tending the stock were English and the great and the good eating at table were Norman French.
In the law we have "buyer", which is derived from the Old English, bycgan, and "purchaser" which derived from the Old French, chacier (meaning to chase or hunt). In common parlance, these words are often used interchangeably. Under the Uniform Commercial Code, however, not every purchaser is a buyer. Division 1 of the UCC defines a "purchaser" as a "person that takes by purchase" and a "purchase" to mean "taking by sale, lease, discount negotiation, mortgage, pledge, lien, security interest, issue or reissue, gift, or any other voluntary transaction creating an interest in property". Interestingly, Division 1 of the UCC does not define "buyer" but does define "buyer in the ordinary course of business". The term "buyer", however, is defined in for purposes of Division 2 (Sales) as "a person who buys or contracts to buy goods".
There are two essential takeaways in this post. First, the UCC defines "purchaser" much more broadly than "buyer". Second, a person may be a purchaser even when she has not given value. Thus, in UCC parlance, the recipient of a gift is a purchaser.
William Shakespeare tended to favor Anglo-Saxon words over words derived from French or Latin. Thus, he uses the word "buy" three times more often than "purchase" in his writings (101 times versus 33). Both words, however, are juxtaposed in these lines from Act II, Scene 1 of Julius Caesar:
O, let us have him [Cicero], for his silver hairs
Will purchase us a good opinion
And buy men's voices to commend our deeds: