In a market where many homes sit, without offers, for months, sometimes years - while a handful of other, similarly sized homes sell, what happens with an appraisal is, the appraiser pulls from similarly sized and located "comps" that DID sell. If your home, in particular isn't one of the "special" homes with undeniable universal appeal, or something that sets it apart from the competition, then it is more likely that in order to sell, you would have to position your home significantly lower than the "appraisal" value in order to attract a buyer.
The question becomes: What are your plans for the property you are interested in? Is it a long term hold? If something drastic and unexpected happened to you...your career..relocation...etc., would you be able to sustain the property as a rental or find a new buyer for it? What kind of risk does this property present to you if you were to pay the seller's asking price?