My friend and I kinda do this. We started 8-10 years ago when we were in high school. We pooled ~10k together to buy legacy staples. Since then we've bought
some cards, but very minimal compared to legacy cards.
We aim for the cards that will be useful post-rotation as many card sale sites do an okay job estimating release price. Also, we mostly played legacy.
Snapcaster for instance was ~$25-30 on release (although some places sold it for as low as 15). Then it was ~20-30 for a while until it saw a lot of legacy play and jumped up. It is currently 30-35. ROI probably similar to an index fund.
Tarmogoyf was probably our biggest 'buy' since our legacy staples. We didn't buy that many (5 play sets). And sold as soon as the price hit $100-120. This we risked due to rumors of 'modern'.
Force of Will, a legacy staple for a long time. 8 years ago we paid ~$10-15 per FoW. Now they're going for ~$90. The S&P is up ~70%, FoW is up ~566%. We bought 20 play sets (80 cards) for roughly $1000.
We're still holding many of our original collection. But, sold off enough to get our original $10k back. At this point the price keeps going up, many cards fall out of circulation (destroyed/lost/etc). The only "risk" is reprints.
One issue is unloading many cards. With stocks I can create a sell order and sell it for roughly asking price. Whether I'm selling $100 of shares or $100k of shares it will go through instantly. With magic it isn't hard to deal with ~500 cards or so. But, once you scale up it is a full time job. If we bought $10k worth of FoW vs $1k we'd have 800 FoW to try and sell at some point. This would take a long time.
You either pay SCG prices to get cards quick. Or you use ebay for a 10-20% discount. But, takes longer and no guaranteed sale.
As far as logistics we each put in roughly 5k and split the original buy cards evenly. Since then our collections have fluxed. He bought/sold a Black Lotus at one point, I never did. He bought individual power 9 cards and sold a 'power 9 set' (5-10% markup) a few times. I was more passive.