I usually keep some lotoed weapons and accessories I find for my next upgrade. Till level 30-35, just keep almost any weapon you find, but try to limit its level as close to the "breakthrough" or "transformation" weapon's level.
For example: if your weapon needs a level 33 weapon to breakthrough or evolve, just keep a row of 29-33 weapons with you.
You can sell the rest. Before you sell them, salvage them. The reason is simple. The weapon's price is always 1 copper (kinda the bummer, right?). When you salvage it, you may get a "broken piece" which sells for 1 copper, BUT you may have the chance to get a special material from the weapon you just salvaged. These rare materials are used in "transmutation" (the 3rd icon from left on the bottom of your inventory). You can either keep them to use them yourself, or sell them in the marketplace. Do the same with accessories.
The "better choice" for you, is to keep blue or purple items in your invenotry (for the puprose of upgrading your items). They give more exp and they cost less than a dozen of green ones. Generally, the higher the rarity, the higher the exp they give when you use them. Purple items are a little "special". When you use them to upgrade your gear, they have a "min" and "max" exp. Needless to say that your weapon will upgade somewhere up to the min and the max values.
As for the soul shields ("bo-pae" for me), it gets a little trickier here. What I do is this:
In case I don't need it:
1) Go to "achievements" > "equipment" > "collecting". Check if the bo-pae piece is listed under any achievement. If it is, wear it to progress the achievement and remove it again.
2) Check the color of the piece. If it's gray, I sell it. If it's green I either sell it or salvage it. Blue and purple always salvage (they give you fusion powder at larger quantities some times).
3) If the blue or purple bo-pae is better than mine, I use it (only as a set though). If they are almsot as good as my current one and have some stats I need, I fuse them.
4) In "fusion" the best stat of each piece is used. So if I have a piece with HP +150, Evasion +6 and Block +30, the "Block" stat will be the one the piece you want to upgrade will get. As far as each piece's stats go, you need to remember this: Piece 1 has worse stats than Piece 8. So if I wanted to infuse my bo-pae set with a critical one, I would find that critical soul shield and fuse it with pieces 5-8.
That's all for now. Sorry for my bad English. If you didn't understand anything, tel me.