It depends on their wants list, at the time...
sometimes, it works to your advantage, very near a tournament or on high-demand pieces...
for a lot of 6-700 units,
they'd be wanting to have a margin,
whoever you do a consignment arrangement with - they're a business also

so they'll negotiate for the lowest per-unit cost,
even if in that aggregate it means you get less for a couple of figures and more for a couple...
I've successful partaken in Paulie's buyback, and in GameHollow,
those were win-wins, which were fair relative to what they could make on the item in a reasonable turnaround time,
and relative to what I wanted

win-win!
and have negotiated but had 'stuff' happen with Troll-n'toad... win-sorta-win?
but hey, I take them at their word that it was a third party not wanting to be in for the minis at that amount.
You'll never hear a gripe about Troll-n'toad's customer service; no matter how small or large an order, they go that extra mile.
So, will you make a mountain of green from selling a large volume of minis at buy-backs ?
no, there are other ways to sell the minis on at breakeven or better, at different times.
Consignment/auction houses, ebay, gumtree (ebay lite)
direct online via whirpool and meetup forums etc...
even the local markets (i've sold many a 'themed squad" plus a tie-in booster over the years - it was great because it was beer money, and they got all their favorite figures --- and I did a buy-back scheme for any pieces I was chasing too. 1 return in 980 odd sales, and that unhappy customer got their full refund and left a happier-yet-disgruntled one).
BUT! big but,
the but is, you can get rid of a large volume of minis that way, and convert them into more useful minis,
or get a discount on a bunch of other stuff.
case in point - D&D pewter figs to Mageknight 1.0, mageknight 1.0 to the next 3rd gen set of D&D and so on...
things that will always command high value,
are the rarer, less produced or more valued pieces - some composite miniatures circa 1920-1945 can fetch,
if intact with porcelain heads, 45 000 USD for a set.
individual figures have been in the teens of thousands... the wilendorf statue? (basically a nude lady carved in stone only the size of 2 SWMinis atop each other) - 600 000 USD in 90s... worth millions at the moment.
even broken 'tin soldiers' can be now worth an inordinate amount - moreso if the packaging accompanies them (and makers-proofs are there, along with hallmarks. The Porcelain composite miniatures are much harder to forge, and hence why they've retained much of their value).
by contrast, things like poly-vinyl SWMinis, which were mass produced and still relatively procurable at the moment,
they'd less intrinsically valuable to many (which is a pity).
I perhaps over-value my SWMinis - but my most prized (and insured), are the custom ones, I have traded for or received from all around the world. They're just that danged cool!

further digression;
thank GOODNESS, buybacks don't work as say, gameshaq or EB,
because, think about it - folks'd suddenly value our minis WAY MORE,
to the point of increasing the prospect of thievery perhaps?
that'd be an awkward as all get-out thing to
a) claim,
b) report to the constabulary...