From this bill of sale,[a]/ we can see that [b]/she had sold the computer[c]/ about six months after she bought it[d].

The past perfect 'had sold' is incorrect in this context. The sentence describes a sequence: she bought the computer, then sold it six months later. Simple past 'sold' is appropriate because the selling happened after the buying, not before another past event. The past perfect tense is used for events completed before another past action.