We have a couple of cars for sale but we have filled in and sent the 'trade' part of the V5. Does anyone know if there is a way to tax the car ourselves or do we need to wait for them to sell and then tax with the new owners details filled out on the V5?

Write name etc on new owner bit in pencil, ahem,

I just took the V5, mot and insure to the PO and taxed it. (Well the wife did)

I know this one!
If a car is registered as "in trade" you can tax it at your nearest DVLA local office. As far as I am aware a post office should NOT do this for you as it is a peculiar dispensation for motor traders (and the DVLA office are able to check the validity of insurance and MOT that most post offices would not do)
Doing this at the DVLA office means the car is not recorded in your name and (obviously) reduces the number of keepers.

One thing I am not certain about but suspect, you might have to have a motor traders insurance policy.

Phil