[midas007]

Build outside-in to validate the business model, so Javascript / HTML5 / CSS would be the first consideration. Node.js is popular for backend because code can usually be ported between the browser-side and server-side, so it's faster to develop and requires less of a learning curve. If it's a mobile app, iOS and Android knowledge. (Focus on one platform at a time, and getting that good enough than doing many.)

Also be aware that double-ended marketplaces are (catch-22's)^2. Scaling this model burns tons of cash before reaching profitability. Please don't put more in than a set amount, say 20k USD and 3 years before trying something else. First-time founders can end up spending between nothing and several million of their own cash before learning whether a business is viable or not... spent cash, blood, sweat and tears don't count for much.

Good luck and remember during the valley/s of despair that it'll work out ok.

[kseven]

Valuable pieces of advice! Thanks a lot