Ya I just got a good deal on 1.5 years old 4x MSI 580s 8 GB.
Will be looking out for more such deals.
And yes Indian banks(most of them); pay 7-8% interest, so basically in 14 years approx, you'll double your money.
I've been doing pretty much the same, mostly grabbing 470s when I find a Sapphire or XFX for 60 to 75 used. Planning to build a couple NVidia rigs after that...and will probably go with new 1660ti instead of used 1070. That will basically tap out all the power I want to use, then I'll start focusing on upgrading tech on the existing rigs.
7-8% doubles in 10.2-9 years. 5% doubles in 14. I need an Indian bank. If I want 8%, it's equities. Fixed & liquid is 1 to maybe 2.