This may be why you are unable to make the purchase.

"We limit sales to ensure we have enough capacity for our customers. We're likely to have capacity available after the initial crunch at the start of the weekend. Players who purchase when we're able to resume digital sales at buy.guildwars2.com will still be able to play for the remainder of the weekend. ~RB2"

Source GW2 facebook page.

https://www.facebook.com/GuildWars2/posts/10150751006819209

You can link your GW1 account to GW2 account to get the bonus items you unlocked from HoM.