So I have a lot of Imperial ships, and I've been thinking of trying out Rebels. I don't have much money, so is there a way I can sell ships/cards at tournaments or something?

Just sell your stuff at a local gaming store. Trade for stuff as well.

Here are a couple of FB groups that swap/sell X-Wing stuff. I suggest reading the rules before you post as they're pretty particular about the rules:

Star Wars Games Swap and Sell

X-Wing Miniatures Swap and Sell (US Only)

I would recommend maybe trying the ships out by borrowing from friends first before selling things off to buy a different faction but I don't really know your life.

3 hours ago, Celestial Lizards said:

So I have a lot of Imperial ships, and I've been thinking of trying out Rebels. I don't have much money, so is there a way I can sell ships/cards at tournaments or something?

I myself am a hardcore Imp player who just branched into rebels. Keep your Imp stuf and jut build your collection slowly.

Grab your Force Awakens core set, Heroes of the Resistance and two more T-70 packs for fluffy x wing fun! or just grab the Corran Horn expansion and fly it alongside Biggs and Asoka untill you get more ships. You dont need a lot, just let your collection grow naturaly, one o two ships every couple weeks.

and proxy EVERYTHING, because you WILL need certain upgrades that you cannot realistically buy immediately

You mostly won't want to sell your upgrade cards, they're largely not faction specific.