that first trailer is similar to the nashbar trailer:
I like the design on the first Ebay trailer since it has some sort of suspension - even though it's limited. While looking at motorcycle single wheel designs, most builders usually added a suspension to their designs to absorb the rode shock and to prevent structural damage. I think the Nashbar trailer would develop stress fractures over time. It has mixed reviews on the Nashbar site. The Nashbar trailer does go on sale periodically.
Nashbar does sell a two wheel cargo trailer similar to the second Ebay trailer:
Again, I am sure it will go on sale.
Looks like the first Ebay trailer is on sale for $86.99 for 5 days.