Emily describes it best--seller subsidy or closing cost credits. As she notes, an appraiser or Realtor can determine the "real" price. But someone just looking at the tax record would see the higher price and be unaware of the amount of seller subsidy.
Other than that . . .
Perhaps selling the property furnished and charging a few thousand or more for the furnishings.
It's also possible--particularly in something like a condo--for the seller to include several months of condo fees in the price. ("Buy my unit and pay no condo fees for 6 months!") The seller would simply raise the price of the condo by the value of the condo fees, and then pre-pay the condo fees.
But there are limits to all that. If the unit doesn't appraise for the higher amount, the financing won't go through. So if there are 3 sold units in the building--say for $100,000, 103,000, and $105,000--a seller would be on thin ice offering the condo at $110,000--even with seller concessions and other items. It might well not appraise for that higher amount. And the buyer's agent very likely would inform the buyer of the other comps in any case, as well as the danger to overpaying.
Hope that helps.