Can a seller sue me for backing out after I've already paid a deposit?

Yes, generally, if you backed out without a legal basis to do so — no unmet condition or valid termination right — that's a breach of the purchase agreement on your part as the buyer. The seller can typically keep the deposit if the agreement makes it forfeitable in that circumstance, and can also sue for additional damages if the deposit doesn't fully cover the seller's actual losses from the failed deal, such as having to resell at a lower price or carrying costs while finding a new buyer.

Whether the deposit is treated as the seller's only remedy or simply a floor against a larger damages claim depends entirely on how the purchase agreement characterizes it — some agreements treat a forfeited deposit as the seller's full and final remedy, while others treat it as a credit against whatever damages are ultimately proven. If you backed out believing you had a valid basis (an unmet condition, for example), that's the key fact to establish, since it changes this analysis considerably.