they are telling me I don't need to pay sales tax when I buy through them.
More correctly, they are not required to COLLECT sales tax from you for an out of state sale. It is an important distinction. When they say "you don't need to pay sales tax," it really means "we don't have to collect it from you." It doesn't mean the tax obligation entirely vanishes.
If the dealer makes a sale within their home state, they are required to report it and collect sales tax from the buyer. If a dealer makes a sale outside their home state, they are (usually) no longer required to collect the sales tax on that.
But, it's not a free lunch. The out of state aspect merely transfers to tax reporting and payment obligation off the seller's shoulders and onto yours. Enter the new term "Use Tax." Your home state probably has a regulation requiring you to report an out of state purchase and pay Use Tax, which is usually almost identical to sales tax. Maybe so, maybe not. It varies by state.
It's true many buyers purchase out of state but never report the use tax they owe. So you have a choice-- comply with the law(s), or ignore that, be a tax cheat, and hope to save some money if it works out?
Finally, you really need to inspect the laws of each state, which is pretty simple to do. Many times, the responsibility for tax payment can get shifted not only by the location of the buyer and seller, but also where the unit is "picked up" or "delivered" to. Once you know the applicable tax laws, you can have the purchase contract defined as you want when it comes to delivery or any other issue that could affect what tax is owed, by who.
Earlier good advice was also to see if there is an ag exemption for your purchase. Good luck and my suggestion is to do good research first .... since individual state laws can vary.