Unless you can prove that the seller knew or should have know about the special assessment and failed to disclose it in the Transfer Disclosure Statement, there's probably nothing you can do except pay the special assessment.
One of the realities of owning a home that is subject to an HOA is that the HOA can increase the regular dues and make special assessments. Homeowners have a duty to pay their HOA dues assessments. California Civil Code §1367.1(a) provides: "A regular or special assessment and any late charges, reasonable fees and costs of collection, reasonable attorney's fees, if any, and interest, if any, as determined in accordance with Section 1366, shall be a debt of the owner of the separate interest at the time the assessment or other sums are levied."
Moreover, notwithstanding more restrictive limitations placed on the board by the CC&Rs, the HOA board may increase regular dues by up to 20% of the HOA's preceding fiscal year without membership approval. (Civil Code §1366(b).)
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, consult with your own attorney.
Mr. Chen is right, of course, but if you can establish that your sellers received notice of the impending special assessment before they sold you the property, and failed to disclose it, you should ask them to rebate the $1,200. If this is the first notice to the homeowners you are probably without remedy. Since it's "only" $1,200, I wouldn't make a federal case out of it. Oh, the joys of living in a property governed by an HOA...
As the other attorneys stated, if the seller knew, they should have disclosed. Even if they didn't and the assessment was for something that happened during their ownership, they might still be responsible. There would be nothing wrong with you asking them to cover all or part of the assessment, but as the other attorney's said, $1200 is not necessarily the amount of money you want to spend a lot of time and energy trying to recover.