Esplen is right, in a manner of speaking. As a general rule, limited time items will only ever rise in price. However, it’s subject to three criteria:
1. The item truly never comes back. This is dependent solely on ANet, who might very well choose to re-release the Aetherblade dungeon down the line, perhaps as part of a Fractal, and the Monocle might become available again as a rare drop there. (There’s also the possibility that a Monocle skin might become available on the gem store, and it looks similar to, or even better than, the current one.) If that happens, prices on the item will plummet and you’ll end up losing most of your investment.
As an example, when the Lost Shores originally came out, some people stocked up on the Tiki skins because they thought they were one-time items. Then, during Secret of Southsun, ANet released the skins again, and the people who hoarded the skins when they first came out basically lost their investment.
I’m not saying the Monocle will DEFINITELY come back, but it’s always a possibility you need to keep in mind when investing or holding on to something to sell.
2. Is there true demand for the item? Back when the Super Adventure Box came out, prices for the Super weapon skins climbed up and up, only to collapse after the SAB went away, and it’s never really recovered since then. The reason behind this was because most people who wanted the Super skins had already farmed them using Baubles; the price of the skins was therefore fueled solely by speculators thinking the price would keep on rising. When SAB went away and it turned out that demand just wasn’t that high after all, the market crashed and now prices for Super skins are about the same as when they were cheapest.
3. The market doesn’t move on. MMO players as a general rule go crazy over new stuff, but when the next new content is released, they largely forget about the previous content. That’s why you often see players wearing/using whatever is the current “flavour of the month”; there’s a lot of people now wearing holographic wings, but I’m willing to bet that 2 or 3 months down the line, you’ll hardly see anybody wearing them.
This often means that demand for exclusive items drops sharply after only a few weeks to a couple of months. At that stage, only die-hard collectors or fashionistas are willing to pay super-high prices for your items, so you might be waiting a long time to get your money, especially if there are tons of other investors (and there will be) trying to sell their own Monocles.
And in that time, maybe YOU are now bored of GW2, or are all excited about another game, and move on, meaning that you will never get to sell your item and thus missed out on enjoying whatever that 42 – 49 gold would have brought you had you sold it while you were still into GW2.
So, for all those reasons, this is why my advice is still to sell an item when it reaches a price where you would be happy with the profit. You could make big money by holding onto the Monocle and selling it further on down the line, but that also exposes you to risk, and it could go either way.
Ultimately, it’s your decision; both selling or holding are perfectly valid choices to make here, depending on what your preferences are.