A LaFrance AKM (full-auto, transferable)
Bought it for $3900; sold it for $4500 a short time later. Within a month of selling it, the prices of F/A AK's jumped to the $6k-7k range because some Krink kits came in the country and people, including myself, wanted to do krink conversions.
The other was a supressed MKII STEn (also full auto, transferable). Bought it to convert to a MK IV Sterling, but when the gun came in there wasn't a spare Sterling part in the US. I even tried the guy who was bringing in rare prototype Sterlings right from the factory where they were made and he couldn't get any. I held onto it for about two months and then sold it. I swear within one day of selling the damned thing, it seemed like MK IV Sterling kits were pouring in the country - I think they were actually giving them away at the local quickie-mart.