Be careful of using a place like eBay because you will get people who sell markers for a living. Literally. They have been doing it for years on paintball forums BST sections. Any good one will have a spreadsheet and know exactly how much to pay to make money off of you. They also will pretend to be a newbie who needs a break on the cost. It's basically the used car dealers of paintball dream palace of cheap deals. Or you get a buyer on their parents account who can't pay or steals your marker.
tl;dr - Use a forum, if it's over 1k in value use a 3rd party, and if it sounds too good to be true it is. Even on the buyer's end. Make a price you want to sell the marker for and a price you have to sell it for and never go under that price. There is always another deal.
Hope this helps you navigate the shark tank that is selling paintball equipment.
Guns are not allowed on eBay in the UK, so it des not realy apply.
Regarding buyers who make their money out of buying & selling on eBay, this can be more of an issue on the forums. There are a few times that people have been spotted on the forum lowballing sales, then quickly putting the items back on sale.
With eBay (selling things other than guns) at least you have the options to set a minimum price, the postage etc and it does not matter how little someone wants to pay as it is an auction. Someone else can bid.
Its up to you as a seller to describe the item well, put on good photos, have a heading that will catch peoples attention and get found in searches. Poor descriptions help the 'professional' sellers to get your item cheap and resell it later with a better description.
A hard luck story will be a rare thing on eBay to get a cheaper price, the only real situation that would apply to is someone contacting the seller to end the autction early and sell at an agreed price. Let the auction take its course