Two possibilities: 1) flapper is leaking. There are several size flappers, despite the "universal" flapper that is sold. 2) the flush valve is either not functioning (not turning off at th proper level), or is set to turn off when the water lever is higher than the overflow tube. Adjust accordingly.

What brand of toilet it? Also, on the inner wall of the tank there should be some identifying numbers. Use it along with the brand name to find the proper flapper. (Example: Home Depot sells the Kohler Wellworth, but when one needs a new flapper, one usually has to go elsewhere to get it.)

If the solution has evaded all your efforts, a service call from a plumber should be less than the cost of a new toilet and installation. Offer $100 for the fix.

One spot many people overlook is the gasket the flapper valve assembly sits on. If it's leaking, the water goes straight down the bowl and the toilet has to constantly add water.

To remove and replace this, you have to take the tank off the bowl, replace the fitting, then replace the tank and reinstall the flapper.

It's not hard. Just don't overtighten the tank bolts because you can crack the tank.

One spot many people overlook is the gasket the flapper valve assembly sits on. If it's leaking, the water goes straight down the bowl and the toilet has to constantly add water.

To remove and replace this, you have to take the tank off the bowl, replace the fitting, then replace the tank and reinstall the flapper.

It's not hard. Just don't overtighten the tank bolts because you can crack the tank.

Another frequent problem is the chain to the flapper being too tight, preventing the flapper from getting a good seal. Conversely, the chain can also have too much slack and get between the seal and its seat.