Experienced vintage pen buyers know to look over prospective purchases with care. Damage and defects will drastically reduce the value of a pen, and they are not always immediately obvious. A magnifier and a light, or a combination of the two, are essential aids.
The condition of the trim should be checked carefully, especially if the pen has been polished so it shines, making brassing easy to miss. Make sure cap bands are not loose or missing. Heat damage can be detected by eye and by feel. The caps of celluloid pens will shrink around the inner cap if overheated, while the barrels may gape around the lever. These are serious defects. Pens can also bend ("banana"), and acceptable restraightening is not always possible. While checking for straightness, feel the barrel for flat spots left from removal of an engraved name. Some claim that they can remove names undetectably, but this is seldom true.
All pens should be checked carefully for cracks at the barrel mouth and the cap lip. Experienced collectors may tell you to run a fingernail inside the cap lip to feel for cracks, but this is a technique that is as misguided as it is widespread. Cracks can best be detected with a magnifier, especially by checking the rim edge-on. By pressing against a cap lip with your fingernail, you are as likely to create a crack as find one. If you insist on jamming your finger into other persons caps, expect to be shouted at and treated rudely by sellers.
Nibs should be checked for loss of iridium. Cracks are also a serious defect, and are usually found extending from the vent hole or lengthwise where the nib emerges from the section. These hairlines are easy to overlook, especially when the nib is dirty with old ink. Scratchy nibs which have had a flat spot worn into their tips can usually be reshaped and smoothed as long as sufficient iridium remains.
Often one will run across pens that have been repaired with nonoriginal parts. This is harder to detect if you are not totally familiar with the pen in question, but keep an eye out for parts that do not belong such as a gold-plated clip on a solid gold pen, or a nickel silver lever on a sterling silver pen. This is a good reason to buy from sellers who are both honest and experienced experts, since many part-time dealers simply don't have enough knowledge to recognize improper restoration, mismatches, or even outright fakes. This is a particular problem with imported items: few dealers in the USA, for example, have any significant experience with vintage Italian pens, while European dealers would be much more likely to overlook nonstandard replacement parts on an American pen.