The safety pens of the early numbered series, a Safety No. 1 for example, and the matching twist pencils such as the No. 10 date from roughly 1920 to 1929. The bodies are generally of black hard rubber, which discolours and grows brittle over the decades. When buying, the main things to look for are therefore cracks, especially at the cap lip and around the turning knob, and an undamaged iridium tip on the nib; re-tipping a point that has been worn down or broken off is laborious and expensive. A good question to put to the seller is what exactly he understands by marks appropriate to the age, since in listings that phrase covers everything from light use to serious damage.
It should also be reckoned with that after so long the seals of a safety pen will practically always need renewing before the pen can be used again. These restoration costs have to be added to the purchase price when comparing one offer with another. As a guide to the price level: a set consisting of a Safety No. 1 and a matching pencil, in condition appropriate to its age, changed hands in 2014 for 343 euros including postage.