keeping in mind that dell uses cheap power supplies as well as having limited upgradeability and a custom designed motherboard you cant replace easily.... i would say its a decent risk. unless you need a new computer badly and dont have much money and the price was really cheap (like $100 or less) then i wouldnt go for it.
now... it is perfectly reasonable to buy a used computer however you dont want to buy anything too ancient. also you need to watch the prices. computers from 2007-2008, especially rather low end computers dont hold much value so should be very cheap. if they are asking any more than what i mentioned its too much.
for $200-300 you can buy a netbook brand new. for $300 you can likely buy a low to mid end laptop that is a few years old (like 2-3).
.... i would post here the maximum you want to spend and if that price includes the need for a monitor, keyboard, etc. or post if a laptop or netbook would be acceptable. also post what you want it for.