When buyers ask "How much did the sellers pay for the house," they are usually trying to determine exactly how much the sellers have in the house, in hopes of gaining an insight into how to negotiate.
MY response is usually going to be that the key issue is what they will TAKE for the house (not what they PAID), and that is reflected by the listing price.
I'm not sure about other states, but Missouri statute does not require disclosure of purchase price. What you may find at the courthouse is how much the mortgage is, but that is not the same as the sales price. Without knowing the terms of the loan (20% down, etc), that figure only represents the amount of the lien.
I know that some MLS systems require disclosure of the actual purchase price, but I see $1 recorded in MLS frequently as a sales price.
The sales price is only part of the equation. Sellers may have made major, costly changes to the house since they bought it. They may have finished the basement, remodeled the kitchen, added a deck or patio, upgraded the bath/s. It is worthwhile to note here that an upgrade is something that adds to the functionality of the house. Repairs are not really upgrades and do not add significant value. While new carpet is nice, it does not add dollar for dollar value to the sales price. For that matter, even upgrades may not add dollar for dollar value to the price, even when they do add value and improve marketability.
How I respond to the question, as is true of many questions, often depends on whether or not I am a buyer's agent. For a client, I will research the MLS history and disclose any information I can find about past sales and current days on market. Comparing a past MLS sheet with the current one and past disclosures with the current one may also shed light on how much the sellers have in the house. Still, though, what really matters is how much the sellers will take for the house. Trying to second-guess that always starts with the list price. Factor in what similar houses have sold for, and that will help you determine what to offer. After all, if your "reasonable" offer is significantly different from the list price, you may need to move on quickly. For help with such an analysis, you need a buyer's agent.