Pro's:
No tax unless you you and the seller are from the same state.
Pick the highest amount you want to spend and if you win it, you won't overpay for it.
Cuts out the pro shop mark up.
Most sellers have very good rating and don't want bad feedback so they will honor your bids.
Con's:
You still need to drill the ball somewhere.
Pro shops don't like you buying online and may rip you a new a-hole with drill pricing.
You may not get the pin and top weights that you desire.
Ball may have damages if the seller doesn't disclose them in auction.
Very high shipping fees.
Ball may get "lost/damaged" during shipping.
You really don't know who you are dealing with. Some sellers are bad. Read feedback.
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3 holes of fun!!