Sportbikes durability like GSXRs are difficult to determine, majority of their owners either ride them hard or crash them hard, you probably will not find a GSXR type moto that have little mileage and the previous owner have only easy touring miles on the engine.
Check the tire/chain/sprocket wear if the moto have been ridden hard.
Check the steering bearing if the moto has been doing wheelies/stoppies.
Check the wheels/brake rotors for round and true if it might have been crashed hard.
Depending on your budget, personally I feel something under $3000 is a good start for a beginner bike, mid/late 90’s Honda CBR600F’s are great beginner sportbikes. Hit up your local craigslist, show up with cash in hand usually good for $50 less on the price for me.
If bike needs new tires -$200 from Bluebook price or whatnot..
If bike needs new brake pads/rotors/fluids -$100-250, need new sprockets/chain, -$120
Check the inside of fuel tank for rust; if engine hard to start when cold/idles funky or @high rpm, carbs may require cleaning.. -$100
Buying used moto from a dealer is probably safer, but do expect to pay 15-30% more than private used moto sales.
As I said before, go to your local dealers and try to sit on ALL of the bikes they allow you to.