No you are ok. Heck uncommon gear in this game is so easy to get with exploration or PvP badges.
I would wait before buying any thing until Lt. Commander though.
And unless you really need that extra little edge in PvP I think you really don't need to buy anything off the exchange. The game is not that gear centric, there is not that much difference in stats between uncommon and rare gear worth the difference in prices that you see IMHO.
But if you do want to make EC. Food for tribble breeding and anomalies/traces for crafting are the way to go at your level.
I sold all my exploration stuff through contracts? Did I sell too low?
anyhow now I have 50K engergy credits what should i buy?
50K is neat for that level. Spend it wisely - probably when you gain a new rank (Lt.Cmdr?) and get access to new ships and a new tier of equipment, you might want to invest in some Mark IV gear.
But having money at endgame actually can have some use, if you want to Craft and don't have enough anomalies, are looking for certain very rare equipment on the exchange, or you want to get a very rare bridge officer with a skill you can't gt otherwise. Not every playstyle actually needs all this, and it might be too early to tell whether you'll do need it or not.
I'm a level 7 ensign engineer.
thanks in advance
Are you really still an Ensign? You should be Lieutenant. You're basically Ensign only for one rank or so. If you#re still Ensign, spend your skill points.
(But I suspect you just miswrote, I think you must first be promoted by Quinn to Lt: before you get any follow-up quests available - like exploration).