Yea you can ask the seller. But, how do you know if the really has equity
arv minus loan price
Using your local county office and/or a title company you can find out what they paid for the property and approximately what they owe on it, then just do the math to find out the estimated equity. I say estimated because there could be factors like them paying more than their monthly minimum, or fees that they accrued through who knows what.
Equity=ARV -original loan -principle paid. If the loan is recent (<4 years) chances most paid is interest still.