kesivas sefer torah - purchasing seforim to fulfill the mitzvah

The achronim (see Sha'agas Arye) question the Rosh's assertion that in our times one can fulfill the mitzvah of kesivas sefer torah through the purchase of seforim. Although the reason behind the mitzvah may be to insure that there are texts available to learn from, the majority of views in chazal hold that the reason for a mitzvah does not determine the parameters of the din, but just adds to our hashkafa. Furthermore, if the purpose of writing a sefer is simply a means to limud haTorah, why is one not yotzei with a sefer that was inherited or written for someone else? The Aruch haShulchan proposes a novel reading of the gemara in Sanhedrin 21 we quoted last week in the discussion of shutfus to address this question. Rava darshens 'kisvu lachem' to teach that one must write one's own sefer and is not yotzei with an inherited sefer. Implicit in Rava's statement are two presumptions: 1) there is a mitzvah incumbent on each individual to write a sefer; 2) the sefer must be written exclusively for each person who wishes to be yotzei. Abaye challenged Rava from a braysa which darshens that only a king must use a sefer written exclusively for himself, to which the gemara answers that Abaye's din refers to the second sefer of the melech. The simple reading of the gemara is that Abaye was challenging only the second presumption of Rava as not applying to a hedyot. However, the A.H. suggests that Abaye was challenging both presumptions - Abaye held a hedyot was not obligated at all to write a sefer! The gemara's answer only defends Rava's first assertion about the mitzvah being incumbent on all, but abandons Rava's second assertion that one is not yotzei with an inherited sefer. Therfore, the Rosh held that one indeed would be yotzei with a sefer written for someone else and inherited, in consonance with the idea that the purpose of the mitzva is simply to have seforim to learn from.