I agree with TMUS, bleed the clutch, to make sure the slave cylinder is producing the entire release stroke to fully relax pressure on the clutch plates and allow them to separate. When you pull the clutch lever, are you pulling it all the way back against the handgrip? An incomplete lever pull might produce the same symptom you're experiencing. If the PO let it sit for a long period of time, such as a year or two without running, the clutch plates may have glazed, which can cause them to stick together. One of my Kawasaki Z-1's does that, and will lurch forward when first is engaged on a cold start if it's been sitting for a while. On that bike before starting the motor, I will pull the clutch lever, place it in 1st and then roll it fore and aft while keeping the clutch lever pulled until the clutch plates break free. Try doing the same thing if bleeding the clutch doesn't help. Another trick is to put the bike on the centerstand, start in neutral, pull the clutch and engage 1st, then engage the back brake. This uses engine power to break the plates loose. If the plates are indeed glazed, replacing the fiber plates (and checking the steel plates for flatness) is the only sure cure.
Good Ridin'
slmjim