When buying glass pens online, it's best to buy NEW pens, unless the seller posts clear pictures of the nib (tip of the pen you write with). With the more thin/slender nibs, there's a chance that it's chipped or a small piece of glass has broken off. This will impair your ability to write with it.
With fatter nibs, the tip is slightly more blunt, which means that it's less likely to have been chipped/broken, and could usually hold more ink.
So, when buying a used glass pen, it's wise to ask the seller about the condition of the nib.