Check online swap markets and online auctions that have a bartering component such as Craigslist.com (check under "For Sale" for the Bartering category), Swapace.com, SwapThing.com, Barterquest.com, U-Exchange.com, Trashbank.com and Ourswaps.com. Check for local bartering clubs. Your local Chamber of Commerce may be able to provide you with information on similar clubs in your area.
Not sure who your friends are? Listen for comments like "You've been a good friend to me" or "It's a fine day with you around." You can also check using the console, just select the NPC in question and enter: player.getrelationshiprank Be aware that not all favor quests result in an increase in your relationship with the person. You can use this mod to help fix some of those relationship oversights.  

Bartering services is best reserved for when you were going to make a purchase anyway. So instead of bartering your dog grooming services for an emergency plumber, you could establish an agreement with your shampoo supplier. You know you're going to need shampoo to run your business. So offer to perform a pet pedicure in exchange for an equal value of shampoo each month.

Trade in Skyrim is oversimplified and uninteresting. A particular type of merchant always has the same amount of gold and offers you the exact same prices as the same type of merchant in another part of the country. Merchants in small towns differ very little from those in the larger cities, and the only factors that affect price are your speech skill and any barter enchantments you might have.
Consider all trade agreements just as you would any other business agreement and make sure you annotate all exchanges. You and your employees will spend time performing services for your trade partners, so there are still expenses to be accounted for, and you want to make sure you get a return on that investment. If the services you receive in return do not result in a net positive for your business, then it likely isn't a good trade arrangement.
Other countries, though, do not have the reporting requirement that the U.S. does concerning proceeds from barter transactions, but taxation is handled the same way as a cash transaction. If one barters for a profit, one pays the appropriate tax; if one generates a loss in the transaction, they have a loss. Bartering for business is also taxed accordingly as business income or business expense. Many barter exchanges require that one register as a business. 
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