No. Normally you cannot sell on Modartt licenses that were bought from users. This should be reflected in the price.

Edit: I will add that the upgraded license is just that, not a second license. Hence you cannot sell the Stage license you bought used as that license is now your Standard or Pro license.
That should really be obvious but wary of how you worded the question..

Well, I'm not sure it's that obvious. E.g., Stage and Standard are, objectively, two substantially different products: one more instrument, more editing capabilities and double the price.

If you can't resell an upgraded license, that decreases the value of a used Stage license to almost nothing to anyone who is planning to upgrade.

Let's say we estimate the resale value of a transferrable license at 30% of its listed price. One could nitpick a bit at that figure, but it's a reasonable one to throw in some calculations.

Since Standard is listed at $269 that means it'd be reasonable to buy a non-transferrable, used Standard license for $188 (-30%).

However, if you buy Stage used at $97 (-30%) and then pay full price for a non-transferrable $130 upgrade to Standard, it ends up costing you $227, that's 39 bucks you'd blow up (30% of the upgrade.)

So the objective price of a Stage license for someone who wanted to upgrade would be more like $58. Take off the $29 transfer fee and the seller would be looking at $29 for their $139 investment, at which point they'd probably rather keep the license for themselves.

Unless Modartt offers a 30% discounted upgrade for second-hand licenses or allows the upgrade to be transferred, that decreases substantially the market value of used Stage licenses, since they can't be transferred and it's not worth it to upgrade them.

Hello,
I am thinking of buying a used Stage 8 License. When I upgrade it, am I'm able to resell it again?

No. Normally you cannot sell on Modartt licenses that were bought from users. This should be reflected in the price.

Edit: I will add that the upgraded license is just that, not a second license. Hence you cannot sell the Stage license you bought used as that license is now your Standard or Pro license.
That should really be obvious but wary of how you worded the question..
There is no upgrade path from Pro. That's the pinnacle of virtual Piano.