As a professional archaeologist, I've always been perplexed by people who assign a dollar value to useless objects from the past.  Perhaps nothing represents this better (for the historical period) than glass insulators.  These things are mildly interesting as they once studded the spars of telegraph poles across the US, but they are the early 20th century equivalent to pull tabs from aluminum cans.  They are ubiquitous on the landscape.  Millions of these things were mass produced with economy of use in mind.  The current (meager) market for these things is contrived.

There is also a lot of ignorance in how they are described. Historical glass in North America falls into a few well-defined categories, almost none of which map onto the ways glass insulators are described on Ebay.  Early insulators are likely to have an amethsyst (light purple) tint due to the use of manganese as a clarifying agent in glass between about 1885 and 1915.  Although manganese made glass clear at the time of manufacture, exposure to ultraviolet rays gradually imparted a purple tint to the glass.  Aqua colored glass used a different clarifying agent that turned the glass clear at the time of manufacture, but imparted a deepening aqua tint over time when exposed to sunlight.  Aqua glass was produced from about 1877 to 1920. During the Great Depression, selenium was used as clarifying agent in glass. When exposed to sunlight, selenium-clarified glass eventually becomes wheat or straw-tinted.

Glass insulators are sort of cool as pieces of the past but be informed when buying.  If selling, be cognizant of the fact that you are probably not selling a unique or highly valued item.

Enjoy glass insulators, if they perk your interest, and try to put them in a realistic context.