What highstreet brand should i buy from?

im buying some jumpers from ebay hoping to get some bargains since the weather is so hot outside. but i want good quality im not sure what to look for. i like some topshop and newlook ones and i found a nice hm one but not sure about the quality. i just dont want them to go saggy or bobbly after washing. anyone have any jumpers from these stores that have lasted?