I am not sure how the buyer was able to pay for the item only if you had a postage charge on your listing ?

Unless it was a random eBay glitch , two possibilities

Are you sure you took the fees into account? (which might explain the 'missing' funds

Was there a select local pick up option perhaps, that the buyer chose instead of postage?

As sandy said,  local pickup may have being selected, or you have forgotten the fee's were taken out.   A 3rd option is it says you accepted best offer,  have you happened to discount by the postage amount in the offer.