One of the most common questions that people ask themselves is who is holding a fountain pen, maybe found in a drawer, seen at a market, given by a friend, is what it could be worth. Giving an answer, however, is very difficult, and the only thing certain, given that the market is always in motion, is that even if you get one, this would not be definitive.
To clear up any misunderstandings, it must also be made clear from the outset that a "virtual" evaluation based only on photographs can never be very accurate. The state of a pen, especially if ancient, can be correctly detected only by holding it in your hand, and observing it carefully (some potentially even very serious defects, such as crystallization, the beginning of cracks, or the consumption of the nib tips) can be detected only with a good magnifying glass and by expert eyes. Anyone who claims to give accurate evaluations from a photograph is not very credible (this can of course be different if you have accurate and sharp high-resolution photos of all parts of the pen).
Also keep in mind that one thing is the evaluation of the sale price on the market directly to the final recipient (the one we are talking about here), another one is the one that can be offered by a retailer, which will obviously be lower because he must still have a margin for his business.
Specified these points we will provide below some general indications to keep in mind in order to be able to make an estimate of the value of a pen, being aware that it will be very approximate anyway.
Pen in production
In the case of a pen that is still in production and trade you are facilitated by the fact that there is a new selling price, just identify it and search on the internet or at a store for the price. If the pen is in perfect condition you can consider its valuation, when sold as a private individual, around 50% of the price when new. Obviously if sold by a known seller, who takes responsibility for taking it back in case of problems, the price may be a bit higher. Even higher if you buy it in a shop, which is bound to provide a guarantee. But none of these prices is applicable to the sale of an item by a private individual.
If the pen is damaged, with small defects or simply worn, the evaluation obviously goes down, but in this case the variables are too many, and the importance or not of certain defects also becomes subjective. The only serious indication that can be given is that the value is certainly less than 50% of the price when new.
The case of limited editions deserves a separate mention, in fact in this case, apart from a few rare exceptions (such as the Hemingway or the Lorenzo de' Medici of Montblanc, whose value has risen) these had artificially exaggerated selling prices in the name of the proclaimed "limitedness" of the edition. But in most cases the number of pieces produced turned out to be much higher than the actual demand, so the end result is that they end up having much lower prices than the list prices today, even when bought in store. In general, in this case, more than the original selling price, it is more appropriate to refer to current prices on the Internet (for which see the last section).
Rating of an ancient pen
If the pen is old, the first step is identification. Usually the brand, and sometimes the model, are written on the pen itself. The make is usually found on the body, clip or nib. The model is sometimes indicated on the body, or on the back (especially if it is identified with a number). If they are not found, it is an anonymous pen that normally has a commercial value very close to zero. If you find indications about the brand (or even the model) you can start to make an identification by searching the wiki on this page, or by asking on the forum.
Another factor to consider is that with an antique pen it is more difficult to give an evaluation, first of all because there is no list price and then because the pen, in almost all cases, will not be in perfect condition. This means that the starting conditions can be very different. Moreover, evaluating the condition of an antique pen is normally much more difficult than for a pen of recent production: there are many different filling systems, materials and processes each of which has its own peculiar characteristics, fragility and problems.
For this reason the condition of an antique pen in many cases can be reliably established only with an expert's examination directly on the object, and very difficult to start from photos (which, although accurate, cannot document the functioning of the internal mechanisms). As an example of how conditions can affect the value of a pen, keep in mind that a beginning of crystallization of celluloid (which is not immediate to identify) reduces what could be a valuable specimen to a future scrap (due to the inevitable degradation of the material) whose value is reduced to that of the few parts that would be saved.
Keep in mind then that in many cases in ancient exemplars there can be parts replaced, reconstructed, not original, missing or recomposed by parts of different pens (you could have a so called frankenpen) so it is not said that all the pen is of the brand found written on one of its parts, or that it is completely original. Identifying this is not necessarily easy and this is a further factor that makes a possible assessment even more uncertain.
Finally, the antique market is much more variable: there are fashions and trends that increase or decrease the prices of certain models, and the evaluations often vary from country to country: for example, Italian fountain pens in Germany do not receive the interest that there is in us, and no one would ever pay, even for the most famous brands, the figures usually required on the Italian market.
How to Search the Internet
That said, you can get a rough idea of the cost of similar items by relying on searches made on auction sites like Ebay or similar. In this regard, here are some instructions on how to do them, related to Ebay which is the most used, but you can follow the same type of path with other auction sites, provided that they provide the required functionality.
In the case of Ebay if you are registered you can make advanced searches, using the homonym link at the top of the page. Enter the keywords (brand and model of the pen) and then select in the following boxes "Sold Items" in order to obtain results related to prices actually paid and not to those requested (which often no one has ever really paid). You also select "private" as the type of seller, lower down, professional sellers in fact normally tick higher prices than a private person, not known in the environment and that does not provide guarantees in case of problems.
You will then get a list of results to examine to see if they have similarities with your pen having a rough indication of the price obtained from similar objects.