Regarding the roof: They used good, dry playwood, with no metal in it:lol.
Dry wood has little impact on GPS signal. It's the moisture in the leafs / tree needles / branches / bark that creates reception troubles under canopy.
Regarding eBay feedback - for big ticket items I agree but for smaller stuff ... I have 16 positive and 1 negative. (my eBay user name is akalinowski) So, IF I was selling anything, you wouldn't buy it off me.
Well, I have low number because I'm not an eBay dealer and only sold few of my personal items. I got a negative because I didn't let one jerk to con me into sending stuff without full payment.
I'd say that buying on eBay, most importantly use traceable method of payment and preferably use your credit card - if things go wrong, your credit card company will stand behind you. Use PayPal. Also, if you are looking at buying new stuff, if it looks too good to be true...
Used stuff from original owners (they usually have low feedback) may constitute the best, real deals out there.
Than again, what do I know about eBay? Not much. I bought sold just few items. Still, my and my friends experience has been mostly positive.
Andrew