Perhaps you have a bullish long term outlook on the future value of some cards.
You would need to speculate on the future of the game.
We can easily assume that common cards will not be great investments because NEW rares, epics and legendaries are likely to make those weak ones useless in the future. You could invest in common while speculating they will be 'necessary' to craft cards with land in the future, but leave that to the splinterlands staff to invest, IMO.
Instead, get in their heads. You have to imagine they want some cards to turn valueable too. So look, which ones are likely to be practically NECESSARY, yet, won't have near-term alternatives 'introduced'?
So you figure some cards out, a handful you want to invest...for the purpose of this, I am going to focus on RARE and EPIC cards. Not to say legendary are any less investment-worthy, but not for the purpose of this post.
To the topic of rares, you have to be extremely sure the card will have value in the future. Like commons, these will be easily diminished with higher rarities in the future. You can also be confident that developer has some idea in his mind to allow a few of these to remain 'neccessary' too.
So this is how you buy them...
You DON'T go buy them all up from say .30 - .40 and wait for more to be listed. If you do this for any amount of time, you'll actually whale the market with very little $.
Instead, you pick out the meat! Buy 80% of them in the range from .30 - .40 and ESPECIALLY the discounted one with high BCX. Always buy those! Leave the singles, and leave 20% of what you buy, right on down the scale.
This way, the market still looks natural, and people will list MORE at the lower price. If you are investing for long term, there is NO RUSH to accumliate RIGHT NOW. Slow and steady, selective. Don't upset the market. Let them list low and you come and snap a few up every now and then.