eBay is interesting. I've actually gotten three GREAT buys on eBay - each instrument was as described and no surprises - two have turned out to be ukes that I play every day.
Still, 90% of sellers know a lot less than the buyers. Things like the lousy old Dixie are often offered at BUY IT NOW prices upwards of $400, which is about 300% too high, and you frequently get folks putting steel strings on old banjo ukes, as one woman said, because "if it didn't have strings on it, people would feel they were getting cheated". They don't know that the first thing you're going to do is replace the strings, bridge and head with new ones immediately. One guy I saw once put violin strings on the Stella uke he was selling - how he got them on there is beyond me. One guy once told me the neck on an instrument stored for decades in a basement was badly twisted (which was visible), but that it played well forty years ago, so no reason it shouldn't still.
Caveat emptor. Just be aware, ask the right questions in writing, and you'll be fine on eBay. No one wants bad feedback.