Others can probably give you a better answer than me, but I think that, even out of the box, I don't think 3a stuff would go down much in value, as long as it's in good condition. There's a good chance you'd even make a profit, what with 3a stuff generally being made on-demand, as it were, and I think Tharg was available in much lesser numbers than other 2000 AD figures.
Whether you are able to sell it in the first place in another thing,* but but you're probably in one of the best place to advertise it!
* I know that statement seems a bit contradictory when considering my first paragraph. What I mean is, I think I've seen such items for long periods of time on ebay with nobody bidding. But if you can sell you should get a good price for it. I'm optimistic with advertising in the right place (here and other communities interested in this stuff) you'll get a good bargain. But don't hold me to that.