I wouldn't say that I wouldn't buy anything but a modded unit, but if all things being equal, I would buy a second hand modded unit. I say all things being equal because sellers of modified units tend to be less flexible on their prices. I do think mods can help a units sound, but it would be hard for me to be the one paying to have the orignal mod performed.
I've been on both sides, the one paying for the original mod, and the one buying the modded unit on AudiogoN. The latter one works best for me. I recently had a choice to make that involved this scenerio. Buy a used Esoteric DV-50S, or a used Exemplar 3910. The 50S seller was much more flexible pricewise, which is probably why his item sold in two weeks while the Exemplar has been sitting there for months.
Now, I could have the DV-50S modded, and I am thinking about it, but I wouldn't spend a whole lot on the mod. I have had trouble selling my modded units (Howard, you got very lucky, you are the exception). I've taken a 70% price hit while selling the two digital players that I had modded. Granted, mods are more popular now than 15 years ago, but The Mod Squad and EAD were two big name modders in the early 90's, and reviewed in Stereophile.
OTOH, I scooped up a modded GNSC preamp for only $300 more than average re-sale at that time. The seller had paid $1300 for the mod. So he got back $300 for his $1300 investment....not good.
So I have no problems buying modded equipment, it's not easy though, because the person who had the mod done is usually not willing to take the 40-50% price hit that it takes to make a sale. Used items usually go for around 50% of retail, mods are no exception. So if the list price on the unit was $2K, and the mod was $2K, expect to get around $2K for your modded unit. Many mod sellers will be looking for $3K. Why do that when I could buy a used unit for $1K and have the mod/warranty done for $2K and have a newer/better unit?
There is no one right answer, but if you do have a modification done, I'd recommend you don't think about re-sale. You should buy it and plan to enjoy it for quite awhile.
John