First, if you bought the Micros system new from a Micros dealer then you either; got the software and lost it OR never got the software, in the later case you are entitled to receive what you paid for. If you never got all of the original software, then request it in writing and send them a copy of your sales contract.
Keep in mind Micros dealers pay nothing for the 3700 software, they pay for the software keys and sell those keys to end-users. Dealers have access to and can load 3700 software onto any pc, it's having or not having a key that makes it of value. So if you did clean your hard drive, it would only cost the "installing" Micros dealer the labor time to re-install the OS and the 3700 software which is likely less than one hour. All you need to do is hand the key over to the buyer.
If your Micros 3700 POS version was ever less than 3.2 then it is very possible there is credit card data on the pc and you should take any necessary steps to protect yourself. If you have a buyer and they have agreed to pay Micros for the re-license and training then have the buyer ask Micros to load the OS & software at the buyers expense. Then you could clean the drive and hand it over.