It comes down to whether your desire to buy the house is greater than their desire to sell. They're under no obligation to hire a mold inspector. But they run the risk of you walking away from the sale and they'd have to disclose that mold was found and fix it themselves anyway. You're also under no obligation to hire a mold inspector, but YOU run the risk of having a mold issue in your new home. This is a decision YOU have to make. Several things could happen.
1. You pay for the inspection, mold is found, you walk from the sale and are out several hundreds of dollars.
2. You pay for the inspection, mold is not found, you buy the home with peace of mind and are out several hundred dollars.
3. You refuse to pay for the inspection and the deal falls apart. An inspection WOULD NOT HAVE SHOWN a major mold problem and you've lost a house you really wanted. You'll never know.
4. You refuse to pay for the inspection and the deal falls apart. An inspection WOULD HAVE SHOWN a major mold problem. You dodged a bullet. You'll never know.
5. You don't pay for the inspection, buy the house anyway and seriously take your chances with the mold problem - maybe it's a major issue, maybe it isn't. If it's a major issue and you bought the house knowing there MIGHT be a problem, you're out thousands of dollars in mold removal services.
When we bought our home, we paid for all the various inspections. WE'RE the ones who wanted them, so we paid for them. The sellers basically said "take it or leave it". OUR desire to purchase this house was GREATER than their desire to sell, so we had various inspections done to our satisfaction and purchased the house. The seller never budged. We still feel we got a great deal because nothing was "unfixable" and the house was priced to sell.
Mold, however, was not one of the issues. My son and husband have mold allergies, so for us, mold WOULD have been a deal breaker and we would have walked had the inspection (that we would have paid for) shown a problem.
At this point, it's a game of chicken. Will you budge or will they? Do you want the house more than they need to sell it? If so, you'll have to pay for the inspection.