That makes sense, they can hold you accountable and would be able to take legal action if you didn't pay. There's no contract here, and an independent seller likely wouldn't want to take that kind of risk. Sure there could be legal repercussions, but that would be very costly for an individual person (as opposed to a big company)... The best thing I could think of would be for the seller to do the sale through Shopify or eBay something of that nature where a 3rd party would handle the payment plan. That being said, that's making things more complex for the seller and adding more fees on their end, making it unlikely for them to want to do that if they can find someone to just pay through PayPal.
That was long winded, but the gist is that you're unlikely to find an independent seller who would do a payment plan based on the honor system. Your best bet is to just save up your money and make the purchase when you can pay in full.