I've gotten some great deals at thrift stores. However more and more I'm finding the best deals are clothes already in your closet with the occasional purchase of something new on sale that is a NEED. In the past year I think my only clothes purchases were: designer sneakers from a thrift store for like 200 dollars -- which they go for 3x that new -- and not very mustachio. Otherwise i bought work clothes: shoes, tie, and belt and a pair of jeans for 20 dollars on clearance. I also bought a pair of boots on clearance from Sears for 20 dollars, but I don't have anything for the snow so that is really a need also.
I don't think these purchases bought used would matter much since I only buy what I need now and on sale. I am the guy who only goes up to the register with one thing and, they are like "that's it?" But I never got these people who walk up with like 20 articles of clothing and have like 20 coupons (they obviously do this a lot and it's obviously a hobby for them). They also never seem to be dressed better than I either.
Perhaps they have different things going on in their lives than you do in your life??? I can think of a number of occasions that I've bought a whole bunch of stuff at one time:
A few years back, I bought a bunch of jackets/blazers at Goodwill, as well as a few pairs of dress pants and a couple of tops. I had moved from a job where I could dress casually all the time, to one where more formality was required. I basically needed a new wardrobe.
This fall, my mother fell and broke her wrist very badly. Between needing tops that she could fit over her cast, and needing pants and skirts that were loose enough for her to pull up with one hand (or pull over her head, in the case of skirts) I needed to purchase "like 20 articles of clothing." Some of which were returned after I took them home for her to try on. She was not at all up to going thrift shopping post surgery, so that's why I did it for her. The small store credit that resulted from the returns was spent on some of the new items they carry periodically, including winter hat/glove sets that I donated for a Christmas drive.
I've gained weight over the last couple of years, and my wardrobe has gradually whittled down to a few pairs of pants that I can wear. So yesterday I went thrift shopping and bought 4 pairs of dress pants (so I can retire my two current well-worn pairs), plus 4 pairs of capris for the upcoming spring/summer season, and 3 versatile tops. Total clothing cost was $65. I kicked myself for not checking the website before I went, to see what coupons were available. I could have knocked another $5 off my total bill!
And a couple of other examples that come to mind:
I've known people who do fostering of multiple kids at a time, long term placements because the kids are special needs. They bought all their kids clothes at the thrift store, to keep the cost low, and because the kids are kind of hard on their clothes, they needed fairly frequent replacement. Absolutely, they would have been carrying armloads to the cash desk, because they'd have set aside a day a few times a year to replenish the kids wardrobes, and if coupons were available, they'd definitely be using them too.
Some people may also be buying and reselling on Ebay or in a more upscale "vintage" store.
These are just a few possible explanations for the phenomenon that you've seen. Hope that helps you see their (seemingly excessive) shopping in a different light!