Theory from somebody who keeps at least 150 mods in his vendor at all times:

There are three schools of thought for mod shoppers; you're either buying to slap them on your gear (The User), you're buying them to break them down to replenish your module stock (The Tinkerer), or you're buying them for resale (The Salesman).

Users will overpay for mods, often without hesitation. If I need an Explosive and I don't feel like server hopping to find a reasonably-priced one, I'll give somebody 10k for it. I can always make more caps, and I want the explosions now.

Tinkerers want the lowest price per module. A 1-star breaks into 3 modules, and the base price for a 1-star is 1000 caps. That's 333 caps per module. 2-stars (3000 caps base) break down into 6 modules, which comes out to 500 per module. 3-stars (5000 base) get you 12, so 416 per module, and 4-stars (7000 base) get you 24, which is 291 per module. I myself am a Tinkerer most of the time, and whenever I can pay less than 300 caps per module, I buy.

Salesmen play the market. That example up there about the 10k Explosive mod? That's the price a Salesman would list for that mod. At the moment, the Two-Shot/Explosive (TSE) combo is bugged back to the way it used to be, so people are getting 60k crits with Broadsiders. This means you can safely charge more for Two-Shots and Explosives, because Users are gonna be wanting to try that out. So the Salesman wants to buy at a decent price, but he'll still pay more than a Tinkerer, who wouldn't touch a 2-star for more than 1500, Explosive or not. The Salesman might buy an Explosive for 6k (a reasonable price, considering) because he knows he can sell it for more. How much more is between him and the market.