There is no reason they can't pay within a day or so.
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You're kidding, right? I'm on my computer all the time, because I work on it. but some people aren't, they actually go about their business, and go a few days without being on the computer. They may have bids in, that end on all different days, but they only have access on library or work days. eBay suggests to make contact within 3 business days..which means if the auction ended on a Friday, you might expect to not hear until Tuesday or Wednesday.
My goal on eBay is to sell and make money. I personally feel if I expected people to pay within a day or two (or even three), I may very well miss out on someone willing to bid much higher, but may read the tos and realize your auction ends on Monday, and their payday is Friday, so there is no sense in bidding. After all, if they see pay within 3 days, they don't know that you are willing to work with them.
I don't see it as being generous when I give them that much time. I see it as my main income, and I want to maximize that. I want to sell the item, not relist it. I often run several hundred auctions a month, and have been doing that for about 6-7 years. I have a folder with my non payers. I just counted them, and it is 19 (although I'm working on another)..not bad, about 3 a year. It could have easily been 200-300, if I was impatient. I must be doing something right.
Even if I only took Paypal, I would give a week, to allow for a payday.
I really believe doing business as I do, cuts way down on NPB, and that means I'm making more money, and not having to relist as often.