An entire small ISP
What can I build from eBay? an entire small ISP in the early 2000s.
We were primarily rural service, so we stocked up on Portmaster PM2s and rack mount 33.6K modems when everyone else was moving to 56K. Those were later upgraded to PM3s (amazing workhorses, they don't build them like that anymore!) when we were able to get inbound T1s (the PM3s coming on the market either due to consolidation of other ISPs, or ISPs that were seduced by marketing of other brands).
Upstream end was a stack of (probably gray market) CSU/DSUs tied into a menagerie of Cisco 2500 series routers (although the Boss could never understand the concept of one 2500 having 1M/1M and a wimpy version of IOS vs. a 16M router with a tricked out IOS image).
Servers were some white box in house machines, a stack of decommissioned Akami boxes, later upgraded to Dell boxes (mostly rackmount, maybe missing some decorative bezels, but functional).
All that was tied together with a batch of 3Com 3c3300 series switches.
Worked great, the biggest obstacle was understanding that if you buy a stack of 4-5 year old servers, in two years you will have a stack of 6-7 year old servers, so you'd better plan on an aggressive plan for server replacement. If you buy brand new equipment, you can coast a bit longer.