They aren't going to want the reputation of being that guild that scammed players to achieve their success. Not fool proof, but maybe don't make deals with guilds that are only like 1 - 10 people. As for collecting shovels.
Literally the only time you are not loosing out on money (be it gems or gold) is when you convert gems into gold after you got every single item that is buyable for gems you want or need in the future. If you convert gems into gold at any point before that, you always convert a higher valued currency into a significantly lower one.
GW2 has the market absolutely right, because everyone in the game participates in the same market. In GW1, there was no way to know what "market rate" was unless you parked a toon in Spamadan and read the relevant GW2 Guru threads (in the game's heyday, you also had to park in GToB, LA, and Cantha).
Trading is a big and important aspect of the Guild Wars 2 economy. Trading between players is possible on the in-game market or via the mail system, making it very convenient to use for players. On the other hand, Odealo is an online player-to-player Guild Wars 2 marketplace which supports trading with real money.