375p seems pretty reasonable these days. The only time I pay 400p for a type5 aug is when I am looking for a magic resist one and just can't find it. Plus, 75p isn't the only profit made by selling them at that price. If the tradeskiller farms his own skins then he/she will be making a much higher profit.
So, if you want to sell them fast and have a steady source of plat coming in, sell them for 375p. If you would rather make that extra 25p per aug, sell them for 400. (I've seen some folks sell them for 450p as well, it might take a few weeks to sell them but they eventually sell).
Either way, if you feel comfortable selling them for 375p (or less), go for it. It's your plat.