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. 

Your basis in the property is lesser of the below in the year it was converted to rental: 1) your adjusted basis - ( purchase price adjusted for other various stuff -- such as settlement cost, improvements to your house and so forth. 2) FMV ( which is not a tax assessment) if you have used the wrong basis, in 2016,  to keep things simpler and practical, I would catch up the depreciation to make it correct this year. your depreciation basis is determined on the year it was converted to rental, and generally, does not evaluate each year.If any improvements are made to a property, they will be depreciated as a separate asset. 

Online and offline.

My background is in online marketing.

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. 

Google online see what you can find.

Go to as many as you can and meet as many people as you can.Keep your connections and reading going here on BP, your networking will be a mix of online and local if you are smart.