just keep an eye out of the for sale section and do a search on completed/archived sales.
People tend to price similarly to each other, so you won't find that much variation - if there is then you can always refer to a previously sold one to try and knock the price down. If it's already quite a bit lower, then ask questions around condition and how well it's been looked after
I would say that the 'going' prices on TP at the moment are:
70-200 f/2.8 IS - £950-£1050
70-200 f/2.8 non-IS - £675-£725
70-200 f/4 IS - £650-£725
70-200 f/4 non-IS - £375-£425
key things to take into consideration are 1) condition, 2) dust inside, 3) sharpness of particular lens & 4) does it come with all accessories &/or box - age is less of a concern if the lens has been well looked after
TP will generally be a little more expensive than places like eBay, but you do get the comfort factor that people tend to be very honest on here and also look after their kit - well worth a few extra £ in my book