Well, I think first of all some rules would be needed.
Like:
A)Motorolafans will provide technical support and software warranty on the phone and its premade mods.
B)Motorolafans can not be responsible for any additional repairs but will try taking the phone to a motorola service centre on the off chance they will make the warranty valid, if the warranty isn't valid the repair costs will be paid by the buyer of the phone.
C)All shipping costs will run on the side of the buyer.
The idea is good, but people should remember that if the phone gets majorly screwed that the only people who might be able to fix it in such a situation might be motorola themselves and that any warranty would most likely be void due to the mods.
And if you're going to make it motorolafans branded, you HAVE to include a motorolafans theme, with a motorolafans logo (which in the case of the linux phones should include some incarnation of tux)
As for smooth functioning, well, you'd have to set a price Admin. Also you'd have to compile a list of mods and programs which can be offered. Also the inclusion of a memory card would be a good option (especially if the buyer wants something that will take a lot of space such as large games or opie).
Once the prices are set you could make a sticky in some forum (maybe garage sales) or in every forum with a link to the area with the price list.
Once people choose what they want they could transfer you the money and e-mail you confirming the transfer in case of any errors like the ones that existed with donations before.
Once the transfer has been confirmed you could then go purchase the phone and mod it.
It would be best if you had one of those tables to calculate the shipping cost, and possibly give the buyer the option whether or not to use land, air, or express mail/courier. (this way there are no forgotten costs or hidden ones).
You could possibly offer the inclusion of certain accesories (screen protectors, silicone cases, etc.), the smaller ones you could just slip in the box before sending it so as to avoid seperate shipping costs.