C string buzzing on 2nd and 3rd fret

Sometimes, an on line vendor will sell a new instrument only to have it returned, after the buyer has tampered with the set up. The right to return goods in on line sales is fully accepted. An acquaintance of mine bought a brand new KoAloha concert model, only to find that the nut had been butchered by some tinkerer - presumably a previous purchaser. She asked if I could put it right. Of course, I could, but I advised her to return it and demand a replacement. I didn't want to get involved in what could turn out to be a contentious dispute.

John Colter.

So, turns out it's not a fret causing my uke to buzz. I went to the local music store and they found out it is the built int preamp/eq/tuner. When playing the C string on 2nd and 3rd fret, it makes it vibrate. Could maybe be because of some frequencies, actually don't know. In fact, it sounded absolutely perfect when we removed the tuner from its place. Obviously they couldn't do anything about it, so I'm returning it to the seller! Quite unfortunate.

Thank you to anyone who gave me opinions, advices and support. I appreciate it a lot, thank you guys.

I'd negotiate with the seller first and ask them how they wish to proceed. It may be a simple fix like a drop of glue. If you like the instrument otherwise and the buzz can be fixed simply then I'd keep it.

Sometimes the causes of tuner/amp vibrations can be traced to the battery compartment or the wiring loom. The battery can set up a buzz if it is loose in its housing. If it is a typical 9V, few wraps of electrical tape around the girth of the battery to make it a light press fit into the housing is usually enough to eliminate this problem. Anywhere the wiring loops contact the inside of the body without being secured is another potential buzzing source. Use the 'chopstick' method (through the sound hole) to move the wires around.
If chasing buzzes/vibrations, it is best to adhere to a systematic check list.
This is how I go about it:
Start at the top and work towards the tail.
Clip on electronic tuner mountings are notorious for buzzing. Temporarily remove the tuner and pluck the offending string/s.
Some geared tuning machines (usually cheaply made ones) can rattle and buzz. Sounds unlikely, but this was on one occasion a most vexing problem to track down.
Poorly fitting, loose, unglued nuts held in place only by string tension, may buzz, but this is rare. One or two sparing drops of glue will fix it.
High frets, low string action, and incorrect neck set are bigger issues that can be referred to a luthier for correction.
'Dry' glue joints between the fret board and sound board can buzz. This may be very difficult to detect. Flex the neck and look for a gap. Some makers do not glue this joint so seeing a gap does not mean that it is a definite problem. Exerting downward pressure over the joint with the end of a ruler while plucking the strings will clarify things.
Wound strings that are starting to unravel can be problematic. This is more common with steel strings.
Poorly fitting, loose saddles can tilt forward in the slot producing sketchy contact. Pack the gap or better still, make a new saddle with a firm sliding fit.
Loose neck fixing screws and/or washers. No need to elaborate on this. It is usually caused by age shrinkage or dehydration of the neck block.
Loose, split or cracked braces. These can be hard to identify and difficult to fix, best left to a repairer or luthier.
Last but not least, loose jack plug nuts and loose metal strap buttons.

Most of these items were gleaned from guitar, but are also applicable to ukulele.
I'm sure to have omitted something, so chip in if you can add to this.