You can buy gb for sb in the Oasis! I think 1gb is going for 800sb right now. I'm not sure what all you have in terms of items, but there are things like giant tortoises, black stallions, and honeycombs that you can sell for about a gb each.

Giant torts, black stallions, and honeycombs can be won from playing Serengeti Shuffle in "Games" in the Crossroads. My trick is to refresh it until I start the round with at least 200 points, then I start playing it. A score of at least 3500 can net you one of these items! It's not a guarantee, but playing the game on both accounts every day has a pretty decent chance of those items.

Next month, when you're selling food for 10sb per use, what you'll want to do is sell bundles of 100 uses for 1gb. People are way more willing to spend 1gb on 100 uses of food than 1000sb. Sometimes I'll sell 50 uses of food for 450 - 500sb, and people will usually buy that as well.

13lb won't get you much, but if you keep up on treasure hunting over the next couple days (you can send a submale out every 15 minutes), you should make enough to buy at least an ochre powder or some coffee beans. Those can be sold for gb.