It will say on the lens ring.
You can find out when it was built. Do a search for Rollei serial number index. Type in your number and a year will pop up. The 'F' started around 1960.
A KEH price isn't too high, depends on year and condition. There are new old stock ones around and they will break $5000 mark.
The Rollei's last and last and last. My 1950's 3,5 E shoots as good as it did new. About once a year you need to exercise the shutter, fire it thru all speeds a couple of times.
Planar gets a better price because it's a Zeiss Planar. Got kind of a Cult following. Rollei folks are as bad as Leica folks when it comes to glass
---------- Post added 11-09-17 at 09:10 AM ----------
PS to give you an idea on price, I paid $400 in 1982 for my 3,5 E, used. The 2,8 was $400 to $500 more at the time, used. Probably an 'F' model.
Planar.....of course