Are there any hardware components(besides HD) on a PC that a hacker can remotely use to get access to your computer?

Hello,

I just got a new PC from a random person on eBay and I immediately replaced the hard drive with a new SSD. Is there any other way (other than installing a piece of software on the old HD) to get access to my PC?

This sounds a bit paranoid I know, but I'm concerned because I may be buying 5 more PC's from eBay so I want to make sure that there isn't any possible way that these computers can be hacked.

Thanks