Note: The commentary below does not necessarily reflect the views or beliefs of Dogcraft.net or its staff.
Short version: My Personal Opinion
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The problem with allowing it to be sold on the store or in shops is there's no way to differentiate between worlds for how long you've been on that server. The elytra is supposed to be an end game item (as far as vanilla exists today) and the ability to purchase it on Day 1 makes the game less vanilla.
And before we get into the "Well all the plugins make it less vanilla," lets look at it this way...The plugins make it easier to maintain a multiplayer server like this one. Teleports are a necessary evil because the map gets spread out over vast areas. Don't want to trudge out hours in the end? Think about traversing hours just to get home because you ended up at spawn because a creeper blew up your bed and you're 30k blocks out. Shop plugins are just there to prevent theft because then it's a he said/she said type of argument. Claims are there to prevent people from destroying your stuff because they thought it was funny that 10 hours you put in was worth 2 minutes of their tnt.
With all that said, the glut of people flying around the end for multiple elytra has calmed over the last 5-6 months. Once you have 5, there's no reason to get more other than to be greedy. Now, if you're raiding the end cities as well as the elytra, then so be it, but flying to a city and taking only the elytra is not really necessary as you only need 5 for the quest.
I would much rather see elytra's banned from being traded, sold, or given away, especially on the new world. This does three things:
1) Alleviates the mods from needing to police these trades/sales
2) Stops the messages in chat of "Why are elytra's 200 diamonds, I don't have that much" (Yes, "Why can't I buy" will come up, but so be it)
3) Eliminates the need to carry multiple elytra. Having 20 elytra that you can't sell/trade is kinda worthless unless you enjoy dying in the void.
Now I'm sure some staff smarter than me have discussed this to their wits end, but that's my opinion on it.
TLDR;
Let's not, they shouldn't be sold/traded at all.
PS: I'm sure there are a few people on the other worlds (mmo, amp, etc) who would have no issues selling you an elytra at 200 diamonds. All that needs to be done is ask if anyone online has one. Another option is to ask players online if they have a home set in the end on the world you're in so you can start off at that point instead of from the end island.