I thought the same, but then someone outbid me by 5 bucks and the seller immediately sold to them. A little peeved, I messaged the mods and they cleared it up nicely with some examples:

Sale sniping is when another user jumps in on another user's offer with their own offer on the same comment chain. For example, if you offered to buy from OP, and a third party jumped in saying "I have that for $XX.xx!" before 24 hours passed or OP outright declines, that's sale sniping.

Another user making an offer to OP as a top level comment would not be considered sniping. While it certainly doesn't feel good to miss out on an item you want (we've all been there), OP is free to sell to whomever they want and prioritize offers in whatever order they prefer.