The short answer is probably nowhere - and definitely not at SongBoom.com.
The re-selling of codes/certificates/vouchers is certainly legal to do (depending on which country you reside in, of course), but the major music labels (and their RIAA/IFPI henchmen) put a lot of effort into threating people that chose to do so. However, this isn't the reason that you can't buy them from SongBoom.com - there is a much different reason.
Some time ago an agreement was made between the owner of SongBoom and representatives at AllOfMP3/AllTunes (MediaServices) in which PIN codes could be bought in bulk. However, instead of delivering the said PIN codes, the company representatives (namely Jack Uspehoff and Andrei Pica) kept the money and cut off all communication. You heard right: AllTunes violated a contractual agreement and stole thousands of dollars from a regular every-day joe.
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