I have used them as a fourth or fifth comp as additional supporting data, but underwriters will balk if a contract of sale is one of the three primary comps. The primary reason is that title has not passsed from Seller to Buyer; it remains vested in the Seller until the terms of the contract are satisfied.
I would also suggest that because no underwriting guidelines are in place, contract sales many times attract those who otherwise cannot qualify for typical mortgage financing, thereby setting the stage for very low down payments or extending payment terms that ultimately may have upward pressure on the sale price.
In my part of the country it's called....Contract for deed or Installment Land Contract and yes they can be used IF it was a recorded transaction. Do you want to use it? It depends. Might be the only comp you have.
These things often reflect issues not typical of the market, i.e., borrower can't get a loan through any other means, seller wants to have a guaranteed income stream without hassel of Deed of Trust, may sell and buy back property, etc. If there's anything else, I wouldn't use it.