Another tactic is demanding a further discount off already rock-bottom prices because they need the money for something else. Food for the kids, petrol to get home, tins of dog or cat food, you name it. This is typically preceded by a pretense that they can't read price tags, and accompanied by an attempt to make it look like butter wouldn't melt in their mouths by saying they don't want the goods for themselves, but for a kid/relative/whatever who might need the info at a difficult time. Listen, love, if you need more money for petrol but can't afford the extra price of a cheeseburger, have you considered cycling?
Oxfam shysters can be of any age or either gender, but they tend to be elderly women more often than not, probably because this is the slice of the demographic best able to pull off the loveably-gaga routine, while at the same time being least likely to get a richly deserved kick in the toothless jawbone from the sort of well-meaning sucker who is likely to be running the shop. Beware the Oxfam shyster.