You 1888S 20$ gold piece (aka - "double eagle") appears to be in very fine to extra fine condition. There may be some remaining mint luster on the reverse so that's a plus. The obverse looks like it may have been wiped or lightly cleaned. Cleaning coins is a no-no as it usually hurts the value.
There were almost 900,000 of this year/mint mintage so it is not "rare" by any means and does not warrant being slabbed at this time.
In my opinion your coin is worth somewhere around the bullion value.























