Personally I wouldn't call it 'theft' per se, after-all, if someone were to perceive a $500 value in the kit you're selling then so be it.
For example, look at the Arabic & Chinese amongst many other cultures where the sales price starts way above what they're often prepared to sell a given item for. There is enjoyment from both sides in the haggling as it's almost a sport & both parties walk away happy in the majority of cases - the buyer as they're happy to have negotiated the seller down & the seller (in many cases) by not having to go to their lowest sustainable price & turning over stock at the same time - if neither of you can agree then you walk away...
If you're desperate to sell, then one way for sure is to sell on price, look at what similar kit goes for & match or beat it...timescales will typically be the quickest in this scenario. You often hear the phrase "it was a steal!" Not that someone stole from you, but the buyer got a screaming deal!
If however, you believe your kit has an edge in terms of condition, minimal use, extras etc & you can wait for a sale, then price it higher & be prepared to negotiate down if required in the event that selling the kit drags on & you're perhaps more motivated to shift it. This strategy often takes longer sell but can net you a greater return.
Either way, just remember a low-ball offer is not meant to be insulting, it's merely an offer to see how desperate you are to sell & for someone to get a deal...you're not compelled to accept any offer, after all it's your kit...
If it were me & selling it to a friend, I'd come from the angle of, "if you look around & want to keep your costs low, for sure you could get similar kit for around the $350 mark plus shipping. Sure mine's a higher price, but mine's in fantastic condition & too be honest, the cheaper kit's got an unknown provenance & I tell you what, I'll walk you through how to use it etc...etc...".
The ball's in their court, there's an offer of a mutually acceptable deal & you've told them they could get it cheaper, make it clear that you won't be offended if they don't want to take you up on the offer & maybe be prepared to come down a little on the price...
Suck it & see I say!