For me money speaks, especially on the classifieds where there can be a bit of messing around with commitment to buy.
It falls on both sides though. The seller should say something along the lines of “yes I accept that offer, but the item will remain openly for sale until I receive the funds”
The buyer should accept this is the case, and if they have the ability to read the classified forum on their phone/tablet/computer then they should have the ability to pay immediately to avoid disappointment.
If sellers accept offers without immediate payment, and buyers delay payment after agreeing to buy then there’s going to be a few times when it goes wrong. People have different timescale expectations, just get it done straightaway in my opinion.