I've been selling on Ebay since 2001. Yes, it is infuriating that people don't read your aucion terms, or choose to ignore them, but the power is all in the buyer's hands these days, as the others have said.
I've given up chasing late payers, really... it causes me more stress and annoyance than it's worth. Most people pay within 3 days, which is what I request. I don't think you're at all unreasonable asking for checkout/contact withn 24 hours but whether or not people *should* respect your rule, most people *won't* and I'm afraid I've given up trying to enforce it - it causes me more headaches than having a few *outstanding* tags on my spreadsheet. 3 days is pretty standard as an Ebay request for contact, in any case.
Like a lot of other people, I ditched the payment by cheque option years ago and I now add a tag asking for no echeques, either - complete pain in the neck! Paypal/instant payment is the way to go for a quiet life. And also in the interest of a quiet life, I never end auctions in the week before payday for most people - you don't get as good a price and payments are always slower.
Some people do need some manners and more consideration but me getting upset with stress (which I can remember doing) over things I can't control isn't going to change them. Ebay really put the nail in the coffin for seller's rights with their 'no negative feedback' policy and it's a buyer's world. My feedback dropped from a straight 100% for 7 years to 99.5% recently over ONE neutral left 10 months ago by a buyer who hadn't read the description properly...not fair is it?
But hey ho ... money's money. Best feeling is making money out of those old bigger clothes, isn't it? How fabulous does it feel to be buying somethng fabulous in a smaller size out of the profits of our old gear?