Dangerous and not legal to put solutions in jugs when it is not labeled fully and properly. Don't risk confusion of the Roundup jugs with Three Way. You risk replacing the lawn and paying a fine, too.
When buying a used sprayer, above are good pointers. Look for oil leaks, and herbicide leaks. oil should be slightly transparent if changed in the last few months. Don't ask, but listen carefully for sellers comments on how carefully it was maintained...if he says nothing about it--be skeptical.
Check his garage--you should find a bottle of 80-90 gear oil which is used to top off and lubricate the gears in the 10 to 1 speed reducer, check the level (for evidence that it has been maintained regularly).
Pressure guage should be steady, needle should not bounce much. Spray some solution back into the tank...pressure guage should drop by about 40 percent when Chemlawn gun is open.
If pump delivers 6 gals per minute--it should fill a 2.5 gal jug in 25 seconds.
Crank up the pressure to about 200 psi--if nothing blows up--you should be safe spraying at normal pressures of about 100 pounds.
Check the pump oil--it should be clean and transparent not milky white like if a diaphragm is blown.