Hi all,
With a heavy heart, I have decided to part with my trusty Leica 8x42BNs and after deliberation, have decided upon the Zeiss 7x42FLs.
In the end it came down to a choice between the 7x42 Ultravids & FLs but the centre sharpness of the FLs (to my eyes at least) made my mind up.
I'm hoping to part-ex my bins at a retailer this coming weekend and think I have a fair trade in price & decent asking price for the FLs.
My main concern now is making sure I come away with a decent specimen - any tips on what to look out for would be greatly appreciated i.e. checking for lint and how to recognise problems with collimation...
Look fwd to reading your replies 
Assuming you are buying from a decent dealer you should not have any problems - but it's well worth checking them for yourself too.
Best way to check for any marks, or bits inside, is to look through the 'wrong end' of the bins (best to look at the sky), amy specks should show up. Make sure you spend sometime looking through the actual pair you are going to buy, any collimation issues (which I'm sure there won't be) should be obvious. It's also important to check that the focus feels right to you, as it seems that this can vary from one pair to another.
If you have any doubts over the bins you are getting talk to the dealer, they should be able to check for marks and collimation if you have any concerns. Just make sure you don't buy them until your happy with them (though I am sure you won't have any problems, the FLs are quality bins), it's worth spending some time and getting it right.
Actually, I would keep the 8x42BNs until you're sure that you're happy with your new bins.
Definitely look at two or three pairs if you're able, and pick the one that looks best to you.