This is a nice domain name, but your target audience is very limited. Not many non geeks even know what telnet is, and SSH1/2 has since superceeded it.
It is nice for the novelty factor and to impress geeks, but i cant see you getting a big company like Van Dykke (SecureCRT) or RemotelyAnywhere to buy it.
It is worth 2,000-2,500GBP to Joe Public.
10,000GBP to a big telnet related company - but i wouldnt hold your breath.
telnet.com is parked ...
I have seen a lot of people suggest "forums" for domains, unless you have some $$$ to spend promoting it or it is an original idea it will be very hard to get many sign ups. Even harder to get any of those to convert to any kind of sales/banner clicks.