Ok, lets try this again.
I can buy things on the web (in Firefox) fine when logged in (to SL in Firefox).
When trying to buy in world (viewer) it would get so far but then pop up an error saying the viewer was not set up for buying things and to try the Marketplace in my browser.
However, when trying to buy this time and get a screen shot, it worked (in the viewer) so I cant show the error here.
So, maybe it was some random thing with the viewer (latest version, Mac, the SL version by the way).
If it should happen again, I will try and get a screen shot and post back here. I just was trying to find out if there was something I needed to set in preferences to buy in world and couldn't find it. I guess not because last time I tried it, it worked.
Hopefully this is a little clearer? Thanks for your time and help. (and no, I'm not being sarcastic, I understand about trying to understand things in a forum.)