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These went from 1 gold to 2-4 gold buy orders in just two days. I am wondering if word is getting out finally about these awesome items. I have a few just from farming but now I wish I invested in them at the start of the escape from Lion's Arch event! I know some people that did, and they are wondering if they should sell. My advice is to sell half your inventory so you make gold now but speculate on the long term value of these as well. Their popularity might drop after the event is over and the next endless tonic shows up (whenever that might be).
If you want a long term market to invest in (assuming that these will not arrive in a future patch, but possibly on the gem store at some point), then I highly suggest investing in Endless Quaggan Tonics. There are several different colors to choose from, but black appears to be rising the most in terms of value.
Have you invested in these or perhaps found a few while farming? They appear to drop from regular alliance supply bags.