In my mind it is totally up to the seller.
If the seller does not specifically exclude dibs in his advert, it does not mean that dibs will stand.
We need to ask ourselves what a dibs actually means. Is it an unconditional offer to buy the goods for the price advertised?
No it's not, all it is is a request to the seller to hold an advertised item for the person making the dibs, until that person decides if he has the funds available, has time to decide if the specific item is worth the asking price, and decides if he actually wants the advertised item or not.
Clearly a dibs has no legal effect at all. How often do we see "dibs" on a for sale advert, only for the "dibser" to cancel his dibs for whatever reason, in which case the seller has no recourse whatsoever.
If the seller gives any sort of undertaking to the "dibser",whether it is on the advert thread, by PM, or verbally over the phone, that he accepts the dibs and will hold the item for him then that is a different matter entirely, and the seller would be expected to uphold his agreement with the dibser. In the absence of such an undertaking by the seller, there is no agreement and the dibs is meaningless.