Updated: Jun 14, 2020
Fossils in general, and trilobites in particular, are rarely in top quality, finest state of preservation when they are found. After all, they have been encased in a rock for hundreds of millions of years, exposed to metamorphosis and the movements of the earth's crust, which ultimately lifted them above the surface of the sea. And exposed to roughly every unpleasantness of geological activity, without being completely destroyed in the process of fossilization.
Most of the finds are only disassembled shell parts and debris. Looking at all the photos of amazing complete trilobites, and finest preserved specimens in different websites and collections online, one might easily get the idea that complete and fine preserved trilobites are common, and that finest-complete preservation is not so rare. Perhaps it might seem so especially in the case of more commercially available trilobite species, in example in the case of ordovician trilobites from Russia, or devonian trilobites from Morocco or Oklahoma, USA. But there is a lot of preselection of that material before it goes into preparation and after... Finding complete good preserved trilobites is an exception everywhere and although in some locations preservation is better, this fact should always be kept in mind:
In the field, at the time of discovery, complete good preserved trilobites are extremely rare!
What (some) collectors and online readers usually do not see - is the selection and countless hours in the field to find a single complete trilobite specimen! Nor the countless preparation hours to reveal a molted part of the exoskeleton only, hiding in otherwise promising looking rock cross section...
Complete all natural good preserved trilobites are rare! Good prepared commercially available even rarer...
Countless hours, days and weeks can be spent in the field without encountering anything other than disarticulated and incomplete or molted trilobite exoskeletons, and the amount of work involved in finding and preparing the good ones cannot be adequately quantified.
However, since fossils are collectibles and thus commodities, there is always a risk (sometimes due to low supply and high demand), that damaged and incomplete specimens are "forced" into a complete condition, to promote sales. Fossils are repaired, restored, supplemented or completely counterfeited. At times one might come to the sad realization that there are more false and falsified items of some trilobite species in circulation than natural finest preserved or simply repaired examples of that same species (for sure true for Scabrella sp. and some rare spiny trilobites)...
Anyway, a clear distinction must be made here: a repair, in the sense of gluing trilobites that were split by the geologist's hammer (during the search), or a minor % restoration of missing shell parts is not a fakery, but a normal procedure in preparation (and often a seal of quality for some species). There is also nothing wrong with the use of some consolidants, which are sometimes necessary, to not impair the stability of the entire preparation. Without the adhesives and consolidants, shell parts could often not be prepared in their actual place.
So even if constant vigilance is often indicated for trilobite collecting, it must not become paranoia (or obsession with 100% natural condition like in megalodon teeth collecting - in trilobite collecting 100% natural state does not exist, unless you collect unglued trilobite cross sections; good preserved shark teeth fossils from well-known USA locations are a lot more common and no preparation required).
What is often referred to as 100% natural and authentic in trilobite collecting - is a trilobite found in a cross section (the only way to find complete specimens is by splitting rocks), prepared out of the rock (matrix) and all parts glued together (and so a crack running through the trilobite and matrix is therefore a good sign of authenticity). The crucial point for trilobite collectors is to be able to distinguish fake, restored and authentic trilobites, but to distinguish between a fast commercial preparation and a slow commercial preparation or a detailed preparation is just as important, because each preparation is by definition a manipulation.