And you buy the computer second hand with cash. You put an impossible to breach password to encrypt the machine. You have kill switch that immediately shuts it down if SWAT storms in. Everytime you go to the bathroom you shut down the machine and of course you log in with a sheet over you like Snowden in Hong Kong. I don't know shit about security really so I'm sure there's additional steps to take to safely post on getbig.
You better wipe the hard drive first.
No need to buy a second hand computer. You can build your own PC from parts. I have always done this. Never bought a PC from Dell, HP, ASUS, etc.
But even if you did buy a pre-assembled computer, you don't have to worry. Install a virtual machine on the Dell/HP PC and use a VPN.
MAC addresses are not sent over the internet. They are used to identify devices within a local network, such as a LAN or WiFi network.
Your WAN is recorded on Getbig. But if you use a VPN it records the VPN address. The MAC address is not recorded.
The FBI and other law enforcement agencies cannot track encrypted VPN traffic, even if they have a court order. However, they may force your internet service provider (ISP) to disclose your connection and usage logs, which will show that you use a VPN. But they can't see what info was transmitted via the VPN connection.
Note, some VPN providers log user activity. Others may claim they don't but actually do. You need to use a provider that does not keep any logs and have been tested thoroughly. If they do keep logs, the FBI could contact the VPN provider and track your identity.