Two instrument styles are commonly played, and they are usually named after the players associated with them. One carries a second gourd mounted at the top of the neck and is heavier, louder and more resonant; the other has no second gourd, is lighter, and is set up with a brighter, more vocal sound. Teachers often have a strong preference, and if you have one, ask before you buy — the instrument is expected to match the tradition you are learning in.
The decorative trap
A large number of sitars sold online are ornaments: correct in outline, painted, and structurally unable to hold tension or be set up. The tells are consistent — no sympathetic strings or a bridge that is not carved to hold any, frets that are glued or nailed instead of tied, a neck with no adjustable bridge under it, and a price far below every real instrument. There is no setup that turns one of these into an instrument, and a repairer will tell you the same thing for a fee.
What to check on a real one
Look at the gourd first, in strong light, all the way round: you are looking for cracks and for repairs, and both are common because a dried gourd in the wrong climate splits. Then the neck-to-gourd joint, which carries the whole tension of the instrument. Then the bridge — its surface should be smooth and even rather than pitted, because that surface is the sound (see the setup page).
Pegs and strings
A sitar has a lot of pegs, and most of them are friction-fitted wood. Turn every one, including the small side pegs for the sympathetic strings — those are the ones sellers forget to check, and a slipping sympathetic peg is a permanent low-level annoyance. Count the strings and compare them against what should be there; missing sympathetic strings on a used instrument usually means nobody has maintained it in a long time.
One practical warning
A sitar is large, fragile, and made from an organic body that reacts to a change of climate. Shipping one internationally is a real risk, and instruments do arrive cracked. If you can buy from someone who will set it up and stand behind it, that is worth more than a better instrument bought sight-unseen — and far more than the difference in price.