It seems from the feedback that they come on the following

(i) After sever reset
(ii) On the first day of the season
(iii) When the market doesn't have many real players on it.

So from (iii) I'm assuming an 1-2 hrs after server reset, there isn't many players online and overnight sales have all begun to end, hence a prime time (and I've noticed myself) for Nordeus players.

So really to get a good chance, you want to find times that people aren't online and selling players much. Apart from mornings, after reset, any other times spring to mind?