What's a reasonable offer for the picoscope?
I don't know if your question is directed at the OP or general.
They don't come up often on ebay.
The 200MHz version 6420D sold for around £700 on ebay recently (I missed it on ebay because the bidding ended whilst I was at work).
The 6430D is currently on ebay from Pico themselves for the (in my view) excessive sum of £4,782 but that is new with 5 year warranty and earlier in the year they were offering it at 40% discount (i.e.£2869).
https://www.ebay.co.uk/itm/124912209765There is an ex-demo 6420D for £1999 but it has been on sale a long time so I think the price is optimistic.
https://www.ebay.co.uk/itm/303217210614At the moment prices seem to be all over the place - I was looking at some of the pico-assured models (when they were on at 40% off) but suddenly all the prices went up by 2/3 - this coincided with pico announcing that the price of new current models is being increased because of supply problems.
The above doesn't really answer the question as to a fair price for a second hand one with no warranty.