I agree with Ron.
There is a technique that some investors have used to minimize the risk of a low appraisal. But it only works if you know which lender will be making the loan on the property. Let's say it's ABC Lending. You contact ABC Lending and ask for a list of their approved appraisers. Then you select one from their list of approved appraisers to do the appraisal. After it's received, the purchase proceeds. And when it comes time for the appraisal, you already have one in hand--done within the 90 days by one of their approved appraisers.
However, I don't know whether that technique still works. And it won't work if the buyers decide to go with DEF Mortgage instead, or if the house doesn't get a contract quickly.
Finally, recognize that what we're all trying to avoid are those crazy, low, uninformed, inaccurate below-market appraisals. If you do a decent CMA, you can be reasonably confident that a good appraisal will come in at about the same number most of the time. The risk is that the appraiser will be from out of town, or rushed, or inexperienced. Getting a seller-paid appraisal doesn't alter those situations. . . . unless you're able to share the appraisal (or at least the comps) with the lender's appraiser.
Hope that helps.