I have also been thinking about this.  It is certainly a great idea.  There are obvious issues with it, but I think it is one of the mechanics that could make rounds more unique and add stategy.

This really makes the choice of getting artifacts or regular gear or cit upgrades more complicated.  Is it worth it to sell most of your gear to deny someone else the chance to get mageslayer?  Would you skip cats to deny someone mageslyer?  If you are in the lead and can sell back mageslayer to get ashkador, are you now giving your opponent access to mageslayer so they can attempt a comeback?  This was discussed lightly in beta, I have always had mixed feelings on it, but think the uniqueness to each match can be increased by doing this.

(To me this is somewhat similar to the idea of randomly allowing only 80% of items in each match, it can add variety, but will it break mechanics and will people think it is stupid?)

I think this would work but just barely. Say you are AA reg and a UB takes ashkandor and mageslayer. Now what do you do. You have 26k gold but you have nothing worthwhile to spend it on and just because a UB was greedy and got kills earlier means he wins? Maybe make it 1 item per TEAM instead of per GAME. This would encourage communication and a variety of builds instead of just health stacking or Ashkandor mageslayer wins.

This would only work if there were more variety.    And really how often are yo uable to get mageslayer and Ank.  I mean really.  Most of the games I play only last long enough to get a mageslayer, and even that is almost impossible due to the need for cit upgrades.  (this is the case often in low death count games.

Well, that's an idea I've also tackled with...

As to :

I would add the following to the OP's idea : when a DG gets killed, he loses all his artifacts (he gets a full refund, but his artifacts are back to the shop).

This would be interesting, but would result in both teams attempting to perma-lock the artifact shop. Depending on who you ask in the community, this is or is not a good thing.

ive seen 2 kinds of comebacks

1. you lock all ports and get creep upgrades

2. team finally extracts head from ass and starts working together.

never seen a comeback due to artifacts, infact the 1. kind of comeback has very often been vs a team with uber artifacts but crap teamwork and portal defense.

to OP

i very much like this idea. also the idea of 80% random shuffle for artifacts.

I agree, no benefit for this. If someone gets artifacts first they ALREADY have a huge advantage by cap locking the artifact flag and tp back to kill every time someone tries to recapture it.(except where arti shop is at citadel).

They have the artifact, so the chances of a non artifact player killing them to remove the artifact would be near zero.