I tested gifting goods to a vendor last night to see what the discount is and try to decide if worth it. I used the Harper ruby, Dragon's Grasp axe and a scroll with default value of 100 for my testing...my CHR is 12 I think.

Default values:
Dragon's Grasp listed as 250 sells to me for 600 = 240% markup.
Scroll listed as 100 sells to me for 240 = 240% markup.
Scroll listed as 100 sells to vendor for 42 = 42% of value.
Harper ruby listed as 1600 sells to vendor for 667 = 41.69% of value.

I gifted 100 gold worth of goods at a time to the vendor and fully left the conversation then re-entered to look at price differences. Prices hit max discount after 200 gold worth of goods was donated(nothing happens if you gift more).

Maxed values:
Dragon's Grasp listed as 250 sells to me for 475 = 190% markup.
Scroll listed as 100 sells to me for 190 = 190% markup.
Scroll listed as 100 sells to vendor for 53 = 53% of value.
Harper ruby listed as 1600 sells to vendor for 842 = 52.63% of value.