What are the arguments for and against the now-widespread practice of selling חמץ prior to פסח?
This idea is old: its origin comes from a Tosefta in Pesahchim: see Aruch Hashulchan 448:16. The Ritva, pesachim 21, is of the opinion that selling and buying back every year is not good. The Shulchan Aruch does not cite the Ritva. One should not say our sales are not real and a trick: see the Chasam Sofer OC 113 about this. Also see the Bach 448.