Can the buyer's assembly of parts sold by the patentee be regarded as infringement?

That depends on whether the patentee owns the patent of the finished product or the patent of the accessory: first, if it is the patent of the accessory, it will not infringe after assembly, and the accessory can be assembled according to the principle of exhaustion, so it will not infringe. Second, if it is a finished product patent, because the accessories are sold, the exhaustion principle (that is, the one-time sales principle) cannot be applied, so the assembled finished product will infringe the patent right.