OP said "unable to sell due to $$ exceeding limit..."The moment victim buys from Merchant1 Person1 IMMEDIATELY Logs off Merchant2 (or Both Merchants)
Victim was unable to sell to merchant2 Since Person1 Logged of his Merchants
In other words, the buy shop had a zeny limit that was lower than the amount it was offering for the item. You wouldn't have been able to "trick" them by selling BBs you already had. OP would have seen this if he'd bothered to check, but he acted rashly due to wanting to make a quick buck by taking advantage of a (seemingly) vulnerable buyer. Just deserts, IMHO.
Follow simple principles like not buying items for double their actual value and you won't be "scammed."