I would like advice or tips on bidding at the online San Diego County Property Tax Sale scheduled for May 2018.

Most people suggest you take matters into your own hands and file a protest online to get your home's tax assessed value lowered. 

Worry about property shopping first. 

Online and offline.

That's interesting because people have been saying that that Zillow was going to be putting Realtors out of business ever since they opened up shop in 2004.

My background is in online marketing.

Rather brokers shop for likely buyers among those who bought from them before.

i find it a lot more efficient to pre-screen online and then have the conversation if everything seems ok upfront.   

Once you have an address of a property that appears to be in distress, you can go to the county assessor to find the owner (even online in some counties) for free (if you don't value your time), or you can use a paid service that will give you all the details right on your phone while you are standing out front, owner or non-owner occupied, value, comps, equity, vacant, even their phone number.

A lot of information can be found online, through books, your local real estate commission, other agents, investors, etc.Focus on learning and making connections.