So I am assuming that he wants the 500K up front? Why can`t you get a loan from a bank? why not talk with your parents? perhaps they might want to help you. Tell them that they can come work with you and you can repay them back as you go.
See, fortunes like these don`t come by very often. I would advise you to walk into a bank and ask them what you need, and what the process is.
SO you are in the enviable position of possibly owning an established business, with a 25 year record of profitability, right?
You mentioned you are in college. Well I have a few avenues you may want to explore that requires work on your part.
Does you college have a SIFE - www.sife.org Students in Free Enterprise program and if so are you a member, if not you may want to (a) join, (b) talk with the program mentor/director and or president. They usually know about unusual sources of funding, especially for student entrepreneurs.
Check with you local business development coorporation/SBA type of entity. Or even the SBA www.sba.gov
Your state may have a low interest loan program or check our www.firstgov.gov
You are a smart person. But do your home work. Get the facts and figures, these should cost you nothing. If the business is for sale then the business records are public. Check building, sanitation and safety records for the past three years. Get accounting records, tax records, any business records and do your home work. Acquire a mentor, preferably someone who has not vested interest in the business, but want to see you succeed, ie one of the business professors at your college.
You want to do educate yourself, so you can educate whoever is willing to finance you. Investors are there, they just want to know that they will get their money back as well as a little extra. My suggestions may not be relevant to your situation, but they should give you some idea where to start.
Oh, one more thing, look into applying for a grant. Again your college business department is a good place to start. Or even a partnership with the current owner, you buying out their % over a fixed period of time.
If the business owner is serious about wanting to sell the business only to you, then the owner will give you a reasonable amount of time to acquire the necessary funding, after all you did not go looking, they owner came asking.