Hardly worth buying anything now except if you buy a 5* players thats close to 6* so he is low-end 5* next season for the bargain hunters. but still for some players they may lack that bit of quality, so I suppose if your buying for reserves for next season its not a bad option. Fast young trainers are maybe worth buying?
Some managers will off load now and some will do it during the beginning of next season, selling players for the maximum amount of $ is touch and go between now (higher starting $, one bid to sell) and the beginning of the new season (lower starting $, but possible bidding wars).