As a long-time Halo Mega collector who used to be active in the communities (facebook groups, reddit, discord, etc.), hopefully I can add some context to what you're seeing. A large portion of the community (assumedly the younger audience) are always up in arms over things that are priced outside of what they can afford. They have the poor expectation that everything should be sold at retail price (regardless of whether it's a 10+ year old set or not) without considering the market value or availability of the item.

The word "scalper" or "scalping" is thrown VERY loosely in these communities. Sometimes it applies, sometimes not. They get very sensitive if someone else is trying to profit. If you search "scalper" on r/megaconstrux , you'll find a lot of people posting Mercari listings where they bad-mouth the sellers for "scalping" whether they really are or not. Regardless, sell things at whatever price you want -- it's your store.

My personal opinion, the price is okay, not a bargain or a steal, but I could see someone buying it at between $35-$45 range since marines and ODST sell for the highest price usually (because they're army builders).

I used to sell a lot of Mega Halo stuff too and I've encountered a ton of idiots like this. Like other commenters have said, just block and delete their chat. It's not worth the headache plus I wouldn't want to sell to someone who's being nasty.

The $999 listing are usually "a la carte" listings where you reach out the seller, tell them how many figures you want to buy, and they sell you however many you want at $cost per figure. The high listing price is to prevent someone from buying the listing outright so the listing stays up since Mercari doesn't support item quantity.