Should I try selling via an estate agent before trying auction?

No, you do not need to do this, this will usually work against you and could taint the property too. Properties that are entered into auction without any prior marketing by estate agents will normally achieve a better sale price; this is because they are fresh to the market, will not have been damaged by any potential over pricing, or sales that have failed to proceed. If your property suits sale by auction, then make auction your first choice.