"If a woman is doing it for money, I doubt she's using a hospital grade electric pump as they are expensive.
Handpumps involve repetitive motion and can lead to repetitive strain injury.
Expressing via pumps is more likely to cause blocked ducts than breastfeeding, and thus increases the risk of mastitis."
There are all sorts of things we do in the course of making money that are potentially harmful. I hurt my back sometimes lugging my work equipment up stairs. If I decide it's bothering me enough to make me not want to do the work - well that's my choice isn't it?
Women may already have a double pump. They can pick them up for very little on eBay.
Or they may just sell a few ounces here and there that they've collected. Lots of women build up a massive stash of spare breastmilk and freeze it. There's a market for frozen and for fresh breastmilk. If they want to sell it - why shouldn't they? It's their's to sell. What do you think they should do with it if their baby doesn't want it or they've switched to giving formula? Pour it down the sink out of principle?