Dedicated HoM Miniature

Is there any reason to keep minis once I have dedicated them to HoM? Can I just trash them?

If you don't want to keep them and aren't interested in selling them (birthday minis are largely worthless when dedicated anyway, though you may get some loose change for gold or green ones) then you can go ahead and delete them. It isn't possible to lose progress in the HoM, barring getting your account permanently banned and thus unable to be linked to GW2.

I have a few purple, but mostly white... is that worth anything? Also I'm really new to the game, not really sure how to sell stuff or find buyers.

If they're birthday minis then no. To trade in game your best bet is to head over to Kamadan, which in the first city in the Nightfall campaign. You can also make threads here, though be sure to read the rules first.

Another good reason to keep a few minis, is to act as a trader break. Minis can not be sold to a trader, so place one mini somewhere close to the bottom of your inventory bags and anything you really want to keep from accidentally merching should be behind and below that.

This way, even if you click sell too fast, the items you do not want to loose will be safe.

That's new. I just "X" off any bags below the top one from sight to the merch myself.

You can give away ded minis to people who want them for good karma and start to build a friends list that way. White are the most common minis, purple is step above but not really worth anything once ded either.

And yeah what Marty said. Kamadan is the trade town in GW1. If you started toons in other campaigns you have to take the cross over quest to get there once eligible which, if new, can take awhile since you have to get to the major port city in each of those campaigns first (KC and Lion's Arch).

Or just create a new toon in Nightfall, do the 10 minutes tutorial and you are in Kamadan right away. That's one of the reasons it is the trade town. Newbies can get there as well in 10 minutes flat.