It's odd but I can purchase ammunition, but not weapons, from Weasel.

It occured last mission but I ignored and played through; the one were you destroy the nerve gas. However, I still can't purchase weapons again. I have enough credits. Any idea what the problem is?

Check how many weapons you have. The Silencer can only carry five guns at any time. You can drop the selected weapon with Ctrl+D.

You can drop any unneeded weapons anywhere in the rebel base, as they are stored to a container right next to the bar.

Yep that fixed it. Thanks.