The simplest method to get this (and the achievement for 250,000 credits worth) is to buy rares from a station and immediately sell them back at the same station.
Cheranovsky City, in the Ngurii system (about 195LY from Sol), sells Soontill Relics which are perfect for just that.
The station usually holds about 80 at any given point. These are have been stated in some Engineering guides as being worth around 19k. At the time of writing this though (June 2023), I found them worth only 17k. So this may fluctuate.
Worst case though, assuming you buy/sell all 80 on hand, as long as they are worth over 12,500 credits, you'll earn your achievements.
This does however require a little bankroll up front (little over a million in this case).
And you won't make any profit (and in fact, worst case, depending on the rare and station, you might loose some credits).
But if you've got in the 10's or 100's of millions of credits already, then profits from rares is probably meaningless to you by now.
Side note: An engineer called Elvira Martuuk wants Soontill Relics. So if you're doing this with these, then hang onto 3.
To make it easier to find: