Whether the seller will be responsible for the subsequent assessment will depend on the state law and the information, if any, disclosed to you by the seller. Typically, states such as North Carolina requre the seller to obtain a resale disclosure packet from the HOA and provide it to the purchaser. If so, did you receive one and what did it disclose about a special assessment? Also, when did the HOA actually make the assessment? If the HOA knew the money would be needed but did not assess it or disclose it, then the seller may not be responsible unless you asked directly and the seller knew and lied intentionally to mislead you--that would be fraud. Your problem in court will be to prove that the seller knew about the upcoming assessment and that you did not.