The skill + amount depends on how many of the other furnishings you have. If you have the 150k/200k items, the stall boosts you up to +5 skill. So +7 total in gear/furnishings, +3 from HQ support, totaling +10 skill.
If you only have the stall, no other furnishings, it's believed to be +3, then +1 for each furnishing you have. That isn't 100% confirmed though. But the +5 example listed in the first paragraph is.
Hope this helps, I'll try to dig up the testing source if you are still skeptical.