Buying my first laptop -- need advice

I would like to buy a used / refurbished Windows laptop. Where is the best place to look for one where I won't get ripped off?

- Local storefront laptop seller in my city?
- Kijiji?
- Computer Forums buy/sell/trade?
- eBay?
- OpenBox?

Specs I'm looking for:
15" (at least 1080p)
Windows with Office installed
500 GB SSD
at least 2 GHz
at least 4 GB RAM
no more than 4 years old

Budget: Under $500.

I don't care too much about brands, but would appreciate suggestions. I have seen a few on Kijiji that match my specs for as little as $150. but I'm not sure how legit that is. What should I expect to pay ?

Thanks!