I'm building an online marketplace, and I'm trying to estimate server costs. I have an idea of how many users I want and by when, but I have no idea how much traffic that will entail.
I personally have had success with the Google Keyword Finder. This is part of the Google Adwords program for estimating keyword traffic and since Google has statistics on everyone's surfing and search habits, this is your best bet.
My advice to you is that before you try estimating server traffic for your marketplace, select 5 to 10 short and long tail keywords that you want to target and then use Google's Keyword Finder to look at the search traffic.
I hope this helps.
One of the machines where I host client sites has been receiving 250,000+ uniques/hour every day for months.
Monthly lease on this machine is <$200 USD/month.
It's more how well your LAMP Stack is tuned, rather than your hardware.
LAMP Stack - Linux + Apache + MariaDB (working MySQL) + PHP + Openssl + CMS (like WordPress) + theme + plugins.
Well tuned + cheap hardware performs far better than poorly tuned hardware with unnecessary bolt ons - CDNs + NGINX + Varnish + Load Balancers (to name a few).
Hire a competent site designer (someone who understands end-to-end site design + LAMP tuning) + likely you're server hardware will be run of the mill.
I personally use OVH for my provisioning. They provide hardware + terminal server interface (for crash recovery) + nothing else. No OS. Nothing else.
A good site designer can turn OVH raw hardware into blazing fast sites.