To argue that capitalist exploitation is inevitable and that it wouldn't be much worse to exploit a sex worker than to exploit a cashier is to miss the point. This is not a pro-sex work argument. It's an anti-capitalist argument. To say that other people also have problems at work does not eliminate the fact that sex workers have problems at work. It just points out that sex workers aren't the only people in need of help. And for the record, I totally agree with that assessment. We all need to re-examine our notions of what work is.
That said, sex work is uniquely exploitative. Cashiers are mostly exploited via verbal bullying, longer work hours with less flexibility, and less pay. Sex workers exploited by market forces would face all of these things as well as the likelihood of repeated violent rape and torture. Do I care less about cashiers than sex workers? No. But I care more about eliminating rape and torture than I care about eliminating verbal abuse.