You will find plenty of pairs of glittery sandals in all sizes upstairs at Jai Patel's shop (just along fron the main entrance to the new Radisson Hotel).
Genuine Levis and some branded sports goods on sale at Saga (cheaper than UK). They also sell genuine crocs there too (only a wee bit cheaper than the UK).
I think that the clothes on sale in the rest of Cavelossim are very poor quality indeed and many of the garments have been on sale there for many years. You can kit yourself out in retro style very easily! You'd probably do better buying a few bits and pieces on the sale rail at Tesco or Sainsburys clothing sections where they always seem to have summer clothing on the sale rail.
Travel light and get your laundry done there - it is very easy to find someone to do your washing and it puts some money directly into the local economy. Ask around at the mini-markets and you will find your own dhobi very easily!
I take very few changes of clothing with me. I still find I have taken too much though - most days are spent on the beach in swimwear and a cover-up; evening clothes get recycled. Unless you really plan on wearing 4 outfits daily, you need very little.
I live in a neutral coloured pair of crocs; put them on when I leave home, great for the beach and for most of the time, occasionally wear a smart pair of sandals if visiting Goan friends at their home or the occasional posh night out at a posh hotel restaurant.
Some of the beach sellers have some not bad casual cotton shirts for sale from time to time - best place is the shacks by the Holiday Inn where there's a very non-hassly seller called Christopher.