Assuming RX300. What year? Did the owner do transmission drain/fill religiously at 15K miles? Do you have credible evidence of a rebuild/replacement and continued conscientious service within the past five years or so? If not, then read this forum carefully again before you pay the asking price...unless you plan to flip it.
Speaking plainly, realistically, and presumptuously: If you simply must have this exact car, I strongly encourage you to factor a transmission replacement or rebuild into the price you pay. (In my area, transmissions for this car are $4,500 and up, no matter who you take it to or whether you rebuild or go remanufactured.) I'd maybe ask about timing belt, but that's chump change by comparison. Routine maintenance. IMO, nothing else you'll ever have to do on this car, short of an engine replacement, will set you back much as a transmission.
So what to offer? Because you won't get the owner to sign it over to you for a dollar, your best scenario will be to try to get him to split the cost of the inevitable repair with you. How badly you want this car will set the upper limit of what you offer. Lacking evidence of a fairly recent rebuild, I wouldn't pay more than $2,000 and only if I really had to have that specific car.
Not that anything's wrong with the RX 300. There's a lot to like about it. I really, really like mine.
But I base my strong statements on my recent experience with buying a carefully-maintained, cosmetically "very good" RX 300 last year. It had 184K miles on it when I bought it. It is "very good" cosmetically inside and out.
Last March, I gambled. And lost. Here's the sad saga:
At 197K miles, seven months after I bought the car, the front transfer case in the transmission began to chew itself apart. I was able to drive the last few miles to my trusted, independent Toyota/Lexus specialist without any problems. But he found several large metallic bits in the pan. Game over.
The evidence I have for "by the book" maintenance history is a lot stronger than what you say you have. My mechanic has maintained my car since 2005. Before that, a local dealer serviced it at all recommended intervals. My guy showed me records of all service he had done, all at the recommended intervals...oil, transmission, timing belt, etc., etc.; all worn/broken parts replaced as needed (except for the power antenna motor). No leaks, except for a small one starting on the rack and pinion.
I even had a receipt from the transmission rebuild done at 75K miles, so it had 110K on the rebuild. I figured any rebuild done by a good shop would last a few more tens of thousands of miles, so I took the chance. Alas, mine failed with 122K miles on the rebuild.
The car is back in service again, and I'm enjoying it very much.
Hope this helps with your decision.