Don't clean any of the glass except with a bit of HAAAAAAAA breath and a soft cloth, even then I would only touch the front glass for now till you've tried it with a torch. If there is any fungus or grot it's likely to be inside anyway.
If the Dignette has a meter then you can check it for accuracy - or if it is a selenium one (no batteries) for any life at all. most of them that had cases tended to survive - the more use they got (light falling on them) the quicker they died. Even if you just compare it to the Sunny f16 rule you can get an idea of accuracy.
Shutters tend to slow down with age as a little grease or oil migrates onto the leaves and this creates a lot of drag when they are opening and closing. Likewise the timing gear (escapement FWIW) that regulates the gap between open and closed can stick, most noticeable on the longest shutter speeds. If these are working well then make sure you mention it. If you close the aperture down you should see if the blade look clean or have any smears - if there are then they'll be on the shutter leaves too.
Focus accuracy is more of a fiddle to check but you can do it on the B setting with something suitable taped where the film goes. Aperture wide open, distance of about 5-6 feet is good, measured from the film plane.
Film testing is the best way to give a buyer confidence but these are fairly cheap cameras so prob not worth your while, they'll sell anyway. 35mm is cheap now, £1 a roll from Poundland, developing for around a fiver at Asda. The 120 you'd need for the Agfa will be a bit more serious money - £5-7 plus £10 upwards to print. Film can be quite forgiving of bad exposure choices though so it's not always as good a test as you might think! Also a small aperture can cover up poor focus accuracy or some lens grot.
Some info here on one of them:
http://www.lomography.com/magazine/1...time-traveller