Good points made by everyone here.
Another big issue that needs to be considered is this: Even if the eBay seller that is offering the GPSR is Mother Theresa, and you have no worries of any kind about the integrity of the seller, you still need to have an above-average knowledge of cartography versions, licencing issues, and so forth if you are buying used GPSRs. Most folks who are buying their very first GPSR don't have this knowledge.
This means that if Mother Theresa offers her SP III with CityNavigator Europe version 4 on eBay, you buy it, it arrives by post the next day, and you later find out that the cartography is 6 years old and the GPSR is slower than molasses in January - there's really squat-all you can do about it.
Michael