Companies may want to barter their products for other products because they do not have the credit or cash to buy those goods. It is an efficient way to trade because the risks of foreign exchange are eliminated. The most common contemporary example of business-to-business barter transactions is an exchange of advertising time or space; it is typical for smaller firms to trade the rights to advertise on each others' business spaces. Bartering also occurs among companies and individuals. For example, an accounting firm can provide an accounting report for an electrician in exchange for having its offices rewired by the electrician.
As Orlove noted, barter may occur in commercial economies, usually during periods of monetary crisis. During such a crisis, currency may be in short supply, or highly devalued through hyperinflation. In such cases, money ceases to be the universal medium of exchange or standard of value. Money may be in such short supply that it becomes an item of barter itself rather than the means of exchange. Barter may also occur when people cannot afford to keep money (as when hyperinflation quickly devalues it).
STOLEN GOODS - Don't expect any favors from Fences. Selling stolen items is going to cost you, so don't expect to get top dollar for ill-gotten goods. Tonila is exempt (actually all Thieves Guild & Dark Brotherhood fences are exempt, but in the vanilla game this just applies to Tonila). Any mod-added merchants who have been made members of the Thieves Guild or Dark Brotherhood faction will also be exempt. Options: -25% to -5% worse prices.
Identify your resources. What items do you have that you could easily part with? Use a critical eye to go through your home, and consider possessions you may have in storage or that another family member or friend is currently using. If you would prefer to offer services, honestly assess what you could provide for others that they would otherwise pay a professional to do. It could be a skill or a talent or hobby such as photography.