Has anyone any advice for the best/worst places to buy a secondhand car in the Basel area.

I would be looking to spend roughly 6 or 7000 CHF and would preferably buy a diesel to save on fuel costs.

Do a search on autoscout.ch, narrow down the search radius to within a few km of Basel/ PLZ 4000.
Without any other details of the type of car you want, other than budget and fuel type, it's difficult to give any really helpful advice! What sort of car do you need? What type of driving do you do?

A budget of 7000 CHF suggests to me that you should avoid any main dealers. You'd also be wanting something with a recent MFK. If you know one end of a spanner from the other, I'd be looking at private sales only. Be prepared to see an awful lot of clapped out rubbish at this price.

Did you manage to find a car this year?
I have a diesel Renault Megane for sale for 6-7K.