I just purchased a hot tub that was set up for 110 to a dedicated 20 amp breaker. But the heater shuts off when the jets are running. The seller says that I can convert to 220 on a 30 amp breaker. Is this possible. Normally a 50 amp breaker is required for a 220 line/hot tub

All depends on the wattage draw of the heaters and then the combined amperage draw, may be 30-50. I had a 120 VAC hot tub but it was only for 3 people. Remember, amperage drops by 1/2 when using 240VAC vs 120VAC, wattage stays the same.

And remember when you rewire something like this you will void the UL listing and the warranty with no recourse if someone ever gets hurt...
If there is something wrong make the manufacturer or sales person make it right...

was set up for 110 to a dedicated 20 amp breaker.
Must be used unit.

the heater shuts off when the jets are running.
Why? Do you mean the breaker trips from over load?

the seller says that I can convert to 220 on a 30 amp breaker.
Who is seller? Does he/she know what he/she is talking about?

Breaker size would depend on amperage draw as Brian said.

It may be possible to wire it for 120 or 240V but I wouldn't unless I could find documentation.

If breaker is tripping you probably only need to increase the cable size, so you can increase the breaker size.

Distance of the hot tub from the breaker panel is also a factor.