Wikipedia states Netcom started in 1988. I’ve had a hard time finding details of exactly what exactly they were selling at that point–when I joined in the early 90s it was a dialup unix shell account with internet access, I assume it was the same when it started, but I’m not sure of that.
I would imagine they sold TCP/IP, but probably only to government approved people. Though by 1989 from what I understand the NSF was being more liberal about who got to go on, definitely by 1990.
We bought dial-up access to a shell account in 1989, from Barry. It was 1200 baud, but we had a modem with an early type of 9600 and he hooked one of those up to a line for us. We had to call long distance from Kansas City to Boston.
I don’t remember exactly, but we had access to a full shell, and could run various Usenet and mail programs like Elm or Pine. I have a box with the stuff from that era, but it’s all on Amiga 1000 3.5" floppy disks.