Selling mats - NPC vs. player prices?

Firstly about selling mats, I notice the in-game material trader... always sells for more than he buys for, even though apparently both are due to player side demand/supply.

Am i right to assume the difference is there because the NPC 'makes a profit' (i.e. as a gold sink)?

Without doing a price check, what's usually a good amount to sell mats to for other players? i.e. should I take the average of the NPC buy price and the NPC sell price?

Or should I try to sell to other players at the NPC buy price?

I'm referring mostly to common mats, if that makes a difference.

The so called "spread" between the selling and buying price is there as a fee or charge for using the trader as an intermediary I guess. It not only regulates the money supply by acting like a gold sink, like you said, but I'd also suggest that it provides a convenient indication of excess supply or demand, facilitating more accurate trader price fluctuation. And yea, for common materials you should just add the buying and selling prices and halve to obtain the value you should sell at.

I've allways used NPC for all mats selling/buying if rly needed em, even ectos and so on.

Overall if u wanna sell to someone else, go around middle between NPC:s buy/sell value