(a) Alex buys one computer for $1,000 and sells another to Clara for $900.
(b) Alex buys the computers for $2,000 without informing Peter and later sells them to
Tom for $2,500.
更新1:
(c) Alex sells the computers to Mary for $2,500 and also receives handling fee from Mary of $100 (because Mary is eager to buy the computers). Alex then gives $2,000 to Peter saying that is the selling price.