Not sure what your budget is, but I have always found the higher the budget the more likely I am to buy from a dealer, if it is just a cheapish banger I will take a punt on something from gumtree or facebook, if I am spending 1000s then the security of buying from a decent dealer and paying the extra is worthwhile. One thing to check is how good the warranty is you get, a lot of them are not worth anything really.
Not sure what your budget is, but I have always found the higher the budget the more likely I am to buy from a dealer, if it is just a cheapish banger I will take a punt on something from gumtree or facebook, if I am spending 1000s then the security of buying from a decent dealer and paying the extra is worthwhile. One thing to check is how good the warranty is you get, a lot of them are not worth anything really.