This is how the scam works.  You will be approached by someone on E-bay.  They will ask you to buy for them.  They will give you the money and you make the purchase.  Or they will ask you to sell a certain item for them.  You are promised a 'cut' and the reasons for them asking you are numerous.

Don’t do it!  You are being scammed.  At minimum they will take your money, but you could also be in the middle of something very, very illegal.

You will probably be paid for your services by some shoddy method such as Paypal.  As mentioned in the Paypal Scams – they can do a chargeback with no consequences.  That way they have the product shipped to them and they do a charge-back, keeping their money.  You have nothing. 

Other variations include you being the middle person between 2 scammers.  The seller(scammer) doesn’t send the merchandise – but instead a brick or some other item to the buyer(also a scammer).  You get stung multiple times.  You paid out money which you never get reimbursed for since seller does a Paypal charge back.  Buyer files a complaint that he got a brick and since you are the only buyer that E-bay sees, you have to reimburse.  If you continue contacting either the buyer or the seller they will find ways to milk you for even more money.