Buy electronic items in city stalls at your own risk

Many Kenyans including me would run to town or any stall near them when they want to buy an item due to the open price that allows bargaining which am forced to think is in Kenyans blood.
Now even the vendors know that and will always state a price of an item and remind you “hio ni bei ya kuongea” (that's bargaining price).

It seems sellers in town have realized that and have come up with a new trick that if you are not careful they will sell you the item/brand you are not interested with and even at a higher price than the bargained one.
I have two friends who fell victims of this trick. One who went to buy a woofer, got to a shop and mentioned the type he wanted, the seller (who are always brokers) quickly asked him the amount he had on confirming they had his woofer on stock. On mentioning what he had they did not even bargain and the broker told him to give him the money to go and pay as he goes for the woofer at the store.
On coming back the broker came with the woofer and an AC power converter which he told my guy he has to buy both or choose another type of a woofer which doesn't use the converter.
Having the receipt and the amount of the woofer going up by Sh2500 my friend had no option but to choose another woofer that amounts to the money on the receipt.
My other friend went to by a TV in the stall and the same trick was used on him buying a different brand and even size.
Somebody will tell me that they won't carry the imposed item, but let me just put you off and tell you this, those guys are very rude and seem to not care. They will rudely tell you to choose another item or leave the money and go.
So if you want to be comfortable with what you buy and to avoid being forced to carry what you are not interested with.
Do not pay until what you choose is wrapped for you, buy your electronics at the supermarket, fixed price stalls or even on the online markets.