I suggest the traders lower their prices on items accordingly to the available amount and decline items under certain value.
Here: The arythmetical decline:
A radio costs, e.g. 1000 caps, if the trader has none of them. Also, he doesn't want any items worth below 200 caps, like shovels or shit.
With every additional radio, the price is lowered by 100 caps.
So, if the vendor has 9 radios, he bids and offers 100 caps for a radio, but hasn't any interest for more, since their price is now under 200. But then, you have a possibility to buy a radio cheap and sell it in another town profitable, depending on interest.
The demand would control the prices.