'Is her personality really that nice~?' 'No way. This time's bad, but back in middle school she was making money selling girls' underwear.'
Literal
''That-much personality [subject-が] good [wondering-かな]?'' ''Impossible. This-time also-[and-だけど], middle-school time [locative-(omitted)] [topic-は] girls [possessive-の] underwear [object-を] sell-off-[て form] making-money-[state-ていた] [reason-し]''
Gossipy character assassination. ありえない ('impossible, no way') is a hyperbolic idiomatic rejection — literally 'cannot exist/happen'. 売りさばく ('to sell off, dispose of by selling') is a compound verb implying getting rid of inventory. 儲ける ('to make a profit, earn') is casual for making money. 中学ん時 is slangy で for 中学の時 ('middle school time').