They have insurance on the vehicle for another 3 weeks and all the quotes I’ve gotten are expensive. Can I just use theirs until I have to get my own?
Once you have signed the title then you now own the vehicle. The insurance coverage from the previous owner is essentially null and void as far as the buyer is concerned so you will need to get your own policy ASAP. Your rates should improve once you have been insured for 6 months, but the best thing you can do to find the lowest rate is to shop with as many companies as possible.