The reason is stablishing two facts: that he wouldn't have sold if not for SR and that the buyers wouldn't have bought if not for SR. Also in he article you'll find why: it's a rebuttal to some defense's argument.

I would have asked the witness the source of the $3,500 before he had access to Silk Road. But it doesn't seem just an arbitrary emotional trick.

Yeah I wonder what other purpose this testimony could have.

Seems to me a moment where a lawyer in the movies would shout 'objection, relevancy' or something.

True, but this is not his trial.

Ideally, no. But it works.