Something is very wrong here....
The agents, the sellers and the buyers SHOULD NOT and COULD NOT influence the appraisals, most specially now that we have very strict rules and guidelines pertaining to appraisals.
Take a look at this Home Valuation Code of Conduct http://www.freddiemac.com/singlefamily/docs/030308_valuationcodeofconduct.pdf
If the buyers are getting a loan, the lender will order an appraisal. It's that lender's appraisal that will trump over any other appraisals that the seller (or any other party, including the buyer) may have ordered separately. The lender and the underwriter will use their own appraisal as the basis for whether or not they will approve a loan for this property, and to determine if it's worth what the buyer is paying for.
If the buyers still want the property, perhaps they can negotiate with the seller for a lower price. But if the seller refuses, and the buyer still hasn't removed the buyer investigation and loan appraisal/approval contingency, the buyer can ask to cancel the contract and get his deposit back.
The seller can put the property back on the market....but there is no guarantee, and it's probably unlikely that the next time it's in escrow, that the next appraisal is going to be much higher or much different than the last one --- not unless it happens many months later when there may be different comps that the appraiser can use.
But wouldn't it be better for the buyer and the seller to try and negotiate a mutually acceptable solution?