Buying specific part variant on BrickLink
I'd say this is something to look out for when dealing with a part that BrinkLink itself notes is very similar to other part numbers.  Reading the detailed notes about buying sets is more important than parts generally.  I'd read a bit more closely about the stated condition of used parts than new parts.

I've made a few dozen orders on BrickLink over the years and I haven't run into this exact situation.  Yet any issues I've had have been easily resolved by contacting the seller.  I wouldn't pester the folks that say they treat them all as the same part, but you can probably find somebody out there who will give you exactly what you want.  Find another seller, read to make sure they don't have a similar warning, and drop them a note to make sure everybody is on the same page.