Abuelhawa v United States  USSC No 08-192, 26 May 2009 deals with whether a buyer of a drug is a party to its sale in a statutory context where the buyer commits a minor offence (or, as they spell it, offense) and the seller a serious offence as indicated by the penalty regime.
Souter J, delivering the opinion of the unanimous Court, said
"Where a transaction like a sale necessarily presupposes two parties with specific roles, it would be odd to speak of one party as facilitating the conduct of the other. A buyer does not just make a sale easier; he makes the sale possible. No buyer, no sale; the buyer's part is already implied by the term "sale," and the word "facilitate" adds nothing. We would not say that the borrower facilitates the bank loan. ...
" ... The traditional law is that where a statute treats one side of a bilateral transaction more leniently, adding to the penalty of the party on that side for facilitating the action by the other would upend the calibration of punishment set by the legislature ..."
He noted that "facilitate" has a similar meaning to aid, abet, and assist.
There has been a diversity of opinion as to whether a buyer is a party to a sale. In R v Meston (1975) 34 CRNS 323 the Supreme Court of Canada held that a buyer who encourages the vendor is abetting the sale.
In the UK the offence is defined as "supply ... to another" and "to be concerned in the supplying ... to another" (s 4 Misuse of Drugs Act 1971[UK]), and it seems to be regarded as "a matter of common sense" that that other person cannot be a person charged: Connelly  Crim LR 296 (CA), although in his commentary to that case Professor Smith said that holding the recipient liable as a secondary party to the principal's offence "might perhaps be thought a devious way to proceed; but it seems right in principle."
In Australia a person can be liable for supply of a drug to himself: Maroney v R  HCA 63.
In New Zealand we seem to say that there is nothing inherently wrong with the idea that a buyer may be a party to the sale to himself: R v Ngamoki 7/11/97, Heron J, HC Palmerston North T5/97, although the better approach may be to classify the buyer as a procurer (who commits the lesser offence): R v Lang (1998) 16 CRNZ 68 (CA).