Ooh, didn't know about the chapter affecting card abilities. But then again I'm only on Chapter 2. >.>
How long do the trade offers at the card counter last? Do they get rerolled when you go on a quest?
I think so? Maybe the last longer, but I don't trade often so I couldn't tell you. I mostly use it to get cards from monsters I haven't had drop before.
Do the expansions you buy from the tailor carry over to other armor?
Nope, just for that armor. The new armor sets you get usually start at the same or higher capacity than the older ones though, so unless you went into heavy duty material farming to expand capacity to it's very limit, you shouldn't have a problem switching between clothes.
Is there any reason to keep crap weapons? I mean is there a way to disassemble them for mats or anything?
Abilities on a weapon are improved once you refine them to +9, and you can then break the weapon and turn the abilities into a card. Personally, I automatically sell anything that has a negative ability, so I'm never short on money since most of the weapons I find now sell for 1000-1500 zenny.