ALWAYS set a time limit for payment on a purchase agreement. 48 or 72 hours is reasonable, or 1 week IF agreed to in advance, which is sometimes needed for a bigger sale.
There are just too many idiots that make purchase agreements and think they don't have to honor them. I got into a big argument here at DNF with a buyer who, after making an offer which I accepted on the condition that he make payment within 48 hours. He simply stopped responding after that. I kept seeing him ONLINE here, so I kept asking what was going on.... 5 days later he said he was ignoring my emails BECAUSE I set a deadline for payment. Apparently some people think you should take you name off the market forever, in hopes that the buyer will someday make payment. This seems to be common these days, and I suspect it's the non-adults mostly doing it... perhaps waiting for their allowance from mommy?
I have also had a seller do this to many on a rather large $$ name.... I made an offer good for 24 hours. The idiot then ignores my emails for the next 5 days, then replies that he was "offended" that I required him to make a decision. Gee, guess I was supposed to leave the $$ sitting in my PayPal account until I die in case he accepts. Now that I'm thinking about it, I should post these peoples's names in the 'people to avoid' section....