I bought bedroom furniture before measuring the room -since it took months for the furniture to bed made, and it didn't even FIT in the room :( so unfortunately, yep on craigslist, competing with all those other prices posted, I took a HUGE loss, threw away about 80% of original price paid. Too much stuff out there for sale and the buyers will not shell out that much money. The buyer hit a goldmine (especially since she was about to buy the exact stuff) and EA does not take returns.

What state are you in?

You might want to try a newspaper ad, but that depends on your area.

What color is it ? and which style sectional is it and what state are you in?

I would contact the seller and see if I could arrange to trade the new furniture in for something else. You might offer to pay a substantial restocking fee, like 20%, or even 30%, or so. Be really sweet and complimentary about their store and make sure to talk to a decision maker. Acknowledge it's a final sale, but explain your dilemma.
Good luck.

This is the chance you take when a sales agreement says: ALL SALES FINAL.

One option is to contact some local upholstery shops, to see if they can cut down the size, Its easier to cut off the depth of a sofa than to cut off width, but on a sectional side that has no arm, then thats fairly easy to cut down. You can cut down the height, easier if its a loose back, but still can be done on a semi-attached or tight back.

Its always been said about upholstery manufacturing, you can cut down but their is no such thing as cloth or frame stretchers!

You may want to check out the Furniture dealers Assn Trade website or Google the Keeping Room in Alexandria, Va classifieds or you may be able to contact a consignment sop.