Today, I was blessed with decent party (Mythic Nerubar Palace) and blasted first 3 bosses. Bloodbound dropped [Bloodbound Horror’s Legplates] which I was excited to roll on, knowing nobody will want it since all plate users already had Mythic 639ilvl pants. Oh boy was I wrong, here came Retribution paladin who needed it, excusing himself that he wants it for TMog(“which is more important as it stays forever, BiS gear is just temporary”). Not only it shares visuals as his set legs, but the audacity to roll NEED for TMOG in Mythic raid is baffling to me. No need to say, that it kinda got under my nerves, but the insolence in the end, when he was willing to exchange it for BoE that dropped for me worth of 330k got me the most. Am I the only one who finds this behavior unacceptable?
To be real, he needed it just to try to sell it. He made that clear when he tried to trade it to you for the boe, since no one else tried to buy it from him.
Yeah might be kind of a jerk reaction, but keep in mind that this is a pug run with people that will not see you again nor do they owe you to pass you gear for your character. Best you can do is just clear the first 2-4 bosses for vault loot, and treat any loot that you win as a bonus (like old personal loot).
No, I don´t find it acceptable. It´s merely one point in an extensive list of reasons never to pugg anything… It was actually the last nail in their coffin, when x-realm gold trading was enabled in Aberrus and the first people rolled on and won trinkets, specifically the OCE, and tried to sell them back to us, we kicked them before the boss’ corpse was cold and closed our raids to randoms permanently. 
there is easy solution which blizzard could implement like ffxiv and it is if your spec has no use for such loot you can only greed or transmog it and not need roll it .
This is already the case and has been since Group loot was reintroduced, if your spec cannot use an item, you cannot roll need on it (except for offspec, which is devalued over mainspec need rolls). The same holds if you have the same item but at an equal or higher itemlevel (note: additional tertiary stat = not same item in game files, a common pain point and shortcoming of ths system… removing tert stats would fix this.)
Either way, the issue of the OP is not be solved by that, because the person that won the item was on a spec that could equip and use it.
But to for example be an “intelligent” loot system that´s able to determinethat it´s not an upgrade for him so as to alleviate this problem, to do that the system would have to run a full sim of every character every time loot they can equip drops. THe resulting overhead would slow everyone´s game to a crawl for about 5-10 minutes after every boss kill while teh server computes who it would be an upgrade for and who not.
It´s good idea in theory, but simply not feasible with today’s technology.
Typical courtesy for organised raiding is Gearing over Transmog, and people needing for transmogs over others gets a swift boot, especially on higher difficulties.
It’s a scummy thing to do when upgrading the lower track unlocks the next track transmog [Heroic is easily on farm for many players already].
On a side note: it’s been suggested lots of times to have transmog rolls separate from item rolls so people can gather appearances without taking items away from those that actually need it. As it stands the Transmog roll is beyond useless…
that is why i have said if you are on a spec that can use it .the need roll should be on .if it is off spec gear you should not be allowed to need it .there is no need fir further checks .
And a paladin can use plate legs in every spec. It would have done nothing to prevent the OPs issue. Again, teh exact system you suggest has been in place from day 1, with the added off-spec functionality.
HARD disagree… if the tanks do not need the shield, but the arms that offtanks /substitutes and tanks M+ for the same people can still use it, why should he have to roll against a mege´s transmog roll? That´s completely bassackwards…
You´re literally making group loot even worse by removing off spec rolls as a level between mainspec need and transmog.
That is how it works though.
If you roll on an item that is for your set main spec role you can roll need on it. You can also roll need if you have your off-spec set and that off-spec can use it (tanks rolling for DPS trinkets) but those rolls are tracked as off spec rolls so they only win the roll if nobody needs that piece on main spec.
You can of course swap over loot spec to change this, but having a healer loot spec set means you will not be able to take trinket drops for DPS or tank, but I think that is fine.
As to OP: It is kind of scummy, yes, but when you do content with pugs this is something that you should expect will happen. In pugs most players are in for their own gains, be it gear, transmog, or just to earn money by taking advantage of the cross-realm trading option added into the game.