there were sold out

第一,主语应该是they吧,不应该是there be结构,否则没主语了。
第二,they were sold out,我们可以这样看：
首先,还原成现在时态 they are sold out。：它们（票们）现在（被）卖完了。
然后把are 换成were,they were sold out。：它们（票票们）当时（过去的当时）被卖完了。
所以were和sold不是都是过去时态,were是过去时态,sold是过去分词表被动语态。