As the main method of trade is barter, and food is the primary currency of Mega Burg, what is considered a prized possession among rats, worthy of trading for? What is it rats most often seek?
Here a lot depends on the status of the rat, and whether he or she comes from the Upper or Lower Tunnels.
Upper Tunnel rats, especially the ruling caste, elders, enjoy anything humans use. Although they might not understand their correct use it is all about the prestige of using such things. For example, a chunk of lipstick can be smeared on their furs, just to give it a slightly different coloration and smell. On the other hand lower caste rats may enjoy more practical luxuries, such as cloth that is both warm and pretty.
Lower Tunnel rats, even those from their own ruling caste, prefer not to use anything too showy. It is considered pompous, unpractical and unnecessary, and would often mark such a rat for a quick mugging. However, unlike the Upper Tunnels, Lower Tunnel rats have a special preference for rat-made goods. They might not be of as high a quality as what the seekers find, but given the choice they would trade for something rat-made.
In few instances where seekers introduced some form of their own currency the payout of such currency would be either for goods traded or services rendered. However, since its the seekers themselves who decide upon the prices of their goods, honest pricing is relatively rare, and hard to manage.
Written contracts are very rare, and most agreements are settled orally.