Theoretically a “buying” announcement must already be tied to one single way to pay, however some announcements’ short description add at least another way to pay.
It enables some to propose a payment either online, such as Paypal, or by bank transfer, and also to declare a large timeout, for example 1 week or so, which is OK for a bank transfer but absurd for an online payment.
Upon starting a deal requiring an online payment the seller logically thinks that the large delay is only enforced if the payment is intrinsically slow, therefore a bank transfer.
However the buyer quickly marks the payment as done to lock the deal, but doesn’t pay and use various excuses to delay until the timeout, mainly asking for details he already obtained.
By doing so the buyer obtains an “option”, because after the long waiting period he can check the change value then chose to either cancel the deal if it becomes uninteresting for him, or to pay and go benefit from an old value. During all this time the seller’s XMR are blocked as he cannot cancel the deal.
The buyer also hope that during this long wait a seller may by mistake release the XMRs, lose his account or whatever.
In my opinion a given “buying” announcement must be tied to one single way to pay.
In any case: a seller must check the announcement and decide whether he can tolerate the max delay.