I have a locksmith going to fix it now. 

In this case, during the move-out day, a rep from our company physically occupied the residence and once tenant was out, made sure that locks were rekeyed and tenant was not abale to even step foot in the unit.  

Reasons that bid exceeds published judgment (amounts for these listed items get included in the lender's bid):Sheriff's cost Sheriff's poundage (a sort of commission to be paid to the sheriff based on dollar amount of highest bid)Transfer taxesProperty taxesMunicipal service liens (utilities and rental licenses for example) or delinquent amountsCode enforcement citationsRe-assessment of judgment (additional payments and interest from actual date of published judgment amount)Property preservation costs (winterizing, lawn care, board up, re-keying, etc. for a vacated property)Additional legal feesThat is not an exhaustive list - but once you start thinking of what gets included into the lender's bid, it becomes easy to see why the lender's auction bid can exceed the published debt / judgment amount. 

When a new tenant comes in, all I have to do is just re-key the door.

This can depend on what kind of response they send to the court, will they put any money into the registry and/or are there any holidays.If the eviction runs its course, at the end the sheriff will come out and you will meet them there with a locksmith and maybe an extra hand in case there are items left in the house.

If your keys don't work, call a locksmith.

if I told you I hired a locksmith.... told him I had just bought it at auction...

We never lock that door.On contacting the landlord was told we're responsible for the locksmith.  

If there is one, then write it down and keep that in a safe place.First house I bought, I had to break in through a window (small town, Labor Day weekend - Locksmith's voicemail said he went fishing LOL). 

I'm that concerned about former tenants rekeying because you have to have the new key in order to rekey.