To me there is always risk of the parts being lost, stolen, or not serviced right. If any of those things were to happen I wouldn't feel comfortable losing half my money in addition to the part not being returned or being made useless.
The way the economy is shops--and others--have been screwed over by customers who never come back with the money. Then what do they do, try to sell your parts that few people want to buy.
I see it differently. The way the economy is I'm not sure if the shop is going to be there tomorrow. Then if there gone with both my part and money guess I get double screwed.