Used i3 makes no sense to me at all.
However comma : used i5-750 and mobo is very attractive as a barebones budget option. Add in PCIe Sata6 and USB3 if you want down the line (never as good as onboard Intel chipset variants in newer chipsets, but they are perfectly functional). I've seen i5-750 board and chip combos sell for $100-$120. Add in an Evo 212+, shoot for 3.8Ghz, and you're well ahead of any i3 in an overall performance package.
http://forums.anandtech.com/showthread.php?t=2050406
Shows i5-750 to score 4.83 @ 3.6Ghz
which is 20% faster than 4130.
That would ONLY apply to people who would use it for the life of the platform, and use the savings down the line to go to an entirely new build (used Haswell by that time perhaps?).
Price-wise, $120 for an i5-750/mobo compares to basically the price of an i3-4130 by itself. Instead of 2C/4T you get 4C/4T. Bearing in mind that 1150 B85 mobos can be had for $70ish, that's $70 one saves. Of course, some of that savings is eaten up by needing to buy a good air cooler, so split the difference and call it a $35 savings.
Pro : better overall performance at 3.8Ghz, better value for $.
Con : older chipset, limited upgrade potential, higher power usage, no PCI 3.0 (meaningless unless you're pairing with a very expensive GPU, and someone going for a $120 combo isn't going to mate it with SLI Titan OC)
Getting used parts isn't for everyone, but for savvy budget users it's often the best path to a well rounded system that's faster than what they could buy with a limited budget when sticking with only new parts. Using common used prices and some new components, here's a typical cheap used build :
Used i5-750 + Mobo $120
New 212+ $20AR
New 550W OCZ PSU $40AR
Generic ATX Midtower $20
New DVDRW $16
New 120GB Samsung SSD $90
New 7950 3GB $180
Result : the fastest sub-$500 build possible without stealing it
